Nikon Flash Use
Using a Single Dedicated Speedlight
Table of Contents
- Camera and Flash Settings
- Basic Flash Use
- Modes set on the Camera
- Speedlight Flash Modes
- Understanding Flash Exposure
- Case Studies
- Usage Notes
- Final Words
Some people approach flash photography by setting their camera to fully automatic green Auto mode and their flash to TTL. Then, pointing the flash head straight forward, they blast away, hoping for the best.
This approach (sometimes called “direct flash”) usually lead to less than pleasing results. It tends to produce flatly lit faces, unflattering highlights on noses and foreheads, menacing red pupils caused by flash light reflected back from fundus at the back of the subject's eyeball, burned out foreground detail, and looming shadows behind the subject.
However, by knowing just a little bit about flash photography, you can make flash work for you, making your flash photographs natural looking with better colours and sharpness than photographs taken under similar conditions without flash.
This note attempt to give you that knowledge. It takes you through some of the basics of a Nikon DSLR camera with a single dedicated Speedlight in the camera's hot-shoe. It covers some of the most useful features of Nikon's Creative Lighting System (CLS), as well as some of the useful modes and features that some dedicated Nikon and third party flashes share with generic flash units.
In this note the focus is on giving you practical advice on what settings and techniques to use for successful flash photography. The theory behind the settings I suggest has been left out deliberately to keep things simple (but will be dealt with later in this series).
I own a Nikon D80 DSLR, the Nikon SB-600 and SB-800 Speedlights, as well as the Nikon version of the aftermarket Nissin Di866 flash. This is also the kit that has been used for testing the examples and scenarios described in this note. I hope that most of the text note is general enough to be useful even if you have a different DSLR or use other dedicated flash units, but there may be omissions and inaccuracies. Additions and corrections are welcome.
Throughout this note, I try to stick to the terminology you'll find in Nikon manuals and literature. For a list of those terms, with a brief explanation, refer to my introduction to CLS.
Below is the definition of some of the other words used in this tutorial:
- Ambient light is continuous light such as natural light (sunlight), tungsten lamps or fluorescent lamps – as opposed to flash light.
- Flash light is a brief, intense burst of light made by a Nikon Speedlight or a similar flash unit.
- Flash key the when flash light is regarded as the dominant or sole light source.
- Fill flash is when the flash is regarded as auxiliary light source to the ambient light.
- X-sync speed is the maximum shutter speed a camera can use that exposes the entire sensor to the burst of flash. On Nikon DSLRs with a mechanical shutter, the x-sync speed is typically 1/200 second (e.g.: D5000, D80, D90), or 1/250 second (e.g.: D200, D300, D700, D3). On Nikon DSLRs with an electronic shutter (e.g. D70, D40), the x-sync speed is typically higher (1/500 second or more).
Modern DSLRs and Speedlights are pretty smart. One the things they come with is a set of built-in rules. Most of rules are put in place to prevent new users from shooting themselves in the foot. However, they may intrude upon your photography. Some may perceive them as design flaws or missing settings.
Below, I list three of the most annoying built-in rules that may intrude upon flash photography, and how you override them.
- The maximum shutter speed you can set with flash is the camera's x-sync speed. This rule is in place is to prevent the focal plane shutter from blocking the light from the flash. You may override this rule by setting Auto FP mode.
- In the exposure modes where camera controls the shutter speed (e.g.: Programmed Auto (P), Aperture (A), and green Auto), the camera underexposes the ambient. This rule is in place to prevent camera shake and movement from blurring the background . You may override this rule by setting slow-sync flash mode.
- The camera's built in flash pops up automatically and fires whenever you snap a photograph, even in places (like a football-arena) where the subject is very far away and the flash is useless. This rule is in place to make sure you use flash in dark light. However, It only happens when the camera is in one of the point & shoot exposure modes. To override this behaviour, select one of these exposure modes: Manual (M), Aperture priority (A), Shutter priority (S) or Programmed Auto (P).
2. Camera and Flash Settings
You can benefit from the use of on-camera flash in all situations. Indoors, outdoors, in bright light, and in dark surroundings.
However, for best results, you need to apply settings to the camera and the flash that matches the ambient lighting conditions of the scene you plan to photograph. Below, the go through the process of picking the settings in dark, fair, and bright light.
The process starts by determine the ambient lighting conditions of the scene. You can measure the light with a handheld incident light meter or with the camera's built in light-meter. With some experience, you will be able to determine the type of ambient light just by looking at the scene.
Method 1: To measure the ambient light with a light meter, just set the meter to ISO 100 and the readout mode to show EV. Position yourself near the main subject and point the translucent plastic hemisphere of the meter in the direction of the camera. Meter, and read the measured EV directly from the light meter's readout.
Method 2: If you don't have a handheld light meter, you can instead use your camera's built-in light meter. Set the camera's metering method to centre weighted and use Programmed Auto (P) mode. Frame the subject you want to meter in the viewfinder. Your camera won't have an EV readout mode, but it will let you know the aperture, shutter speed and ISO for correct exposure. When you know these three parameters, you can compute the EV number with the following formula:
EV = log2(Nē/S/(ISO/100))
(N = aperture (f-number), S = shutter speed)
Method 3: Set the camera's metering method to centre weighted and use camera to Aperture Priority (A) mode, set the sensor speed to ISO 200, and set the aperture to f/4.0. Frame the subject you want to meter in the viewfinder. Read the resulting shutter speed from the viewfinder display.
You should now be able to use the table below to determine the type of ambient light you have to deal with. If you used method 1 or 2 to meter the light, refer to the EV-number in the first column. If you used method 3, refer to the shutter speed listed in the second column. In both cases, you will find the type of ambient light in the third column. The last column is contains descriptions of some typical scenes where you will find this type of ambient light.
|< EV 7||< 1/15||dark||interiors lit with tungsten, stage shows, fairs|
|EV 7 - 10||1/15 - 1/125||fair||interiors lit with fluorescent lights, ice shows|
|> EV 10||> 1/125||bright||subjects outdoors in the daytime|
Below you will find suggestions for what initial settings you should use for camera and flash in each of the three lighting conditions.
Dark Ambient Light
When it is dark enough, adding light to the scene is probably the only way you can get a decent photograph. While dark ambient light is by no means the only lighting conditions you should use flash, it is probably the condition where most people more or less automatically will turn on their Speedlights. Here is how to best use a single camera-mounted Speedlight when there is very little ambient light.
In dark ambient conditions, the camera's built-in exposure system will pick very slow shutter speeds to expose correctly for the ambient light if we let it. We don't want this. Instead, the power of the flash will be used to control exposure. This means that the flash will be key and essentially the only light on the subject. The ambient will contribute only to the background exposure.
- Initial settings:
- Camera: Manual (M), f/4.0, 1/60 second
Flash: TTL or Auto mode
Auto FP: Off
Slow sync.: Don't matter
In dark conditions, we don't care much about the ambient, so we rely on the TTL mode of the flash for exposure control. This is why we select the Manual exposure mode on the camera.
While I prefer TTL in dark conditions, some says that the non-TTL Auto mode (only available on some flash units) works even better for exposure control in this situation.
In dark conditions, the power of the flash will control the exposure of the main subject no matter what you do to the ISO, aperture, or shutter speed. This means that you can change ISO, aperture, and shutter speed from those suggested without impacting on exposure. Nikon's CLS will give you a correct exposure as long as your main subject is within the range of the flash.
The ISO setting determines the range of the flash. To extend the range of the flash, increase ISO.
The aperture will primarily control depth of field. Open up the aperture to decrease depth of field, increase background exposure, and increase flash range. Close down the aperture to increase depth of field and decrease background brightness. Stopping down will also reduce flash range.
The shutter speed you select will only control the background exposure. Decrease the shutter speed to brighten the background. However, doing this increases the risk of motion blur impacting on the background. Increase the shutter speed to darken the background. You should not increase the shutter speed beyond the x-sync speed without turning on Auto FP. Note that if you do so and and set at shutter time faster than x-sync, the flash range will be significantly reduced.
When the camera is in the Manual exposure mode, the Slow sync. flash setting don't matter because it is you, and not the camera, that set the shutter time.
Bright Ambient Light
In bright light conditions the cameras built-in automatic exposure control will give a correct exposure for the ambient light. In this case, we only want the Speedlight to add fill flash to light up the shadows.
- Initial settings:
- Camera: Programed auto (P)
Flash: TTL BL mode (or TTL mode)
Auto FP: On
Slow sync.: Off
If you have a Nikon Speedlight, you should use TTL BL mode to allow CLS to automatically balance the subject with the ambient. If you use a dedicated third party flash, the TTL mode will have to do.
The initial setting relies on the Programmed Auto mode, which let the camera pick the initial settings for aperture and shutter speed. However, you could also use Aperture Priority if you want to control depth of field, or Shutter Priority if you want to control motion blur. All these settings will expose for the ambient, and add fill flash to brighten the shadows.
The flash will be used to fill in the shadows with just enough light to balance them with the background. This only makes sense if your main subject is less bright than the background. If your main subject is brighter than the surroundings, there is no point in using fill flash.
Fair Ambient Light
While the two extremes (dark and bright) are simple to deal with. The in-between situation, where there is some, but not very much ambient light, more complex to deal with.
In this situation, we say that the ambient light is “fair”. and this is when it too much ambient light to make flash key the obvious choice, and too little for us to be able to get good results with flash fill.In fair ambient you have to make several more decisions to determine the initial settings. You have to decide how you want to shoot:
Do you want the flash to be dominant? CAN you even make the flash dominant? Do you want the flash and ambient to balance on the subject? Do you want the flash to be only light Fill and the ambient primary? Do you have to turn off the flash and shoot available light?
Then you might ask, why would I want the flash to be dominant?reduce the effect of mixed lighting
Well, one reason is to control colour. The flash has roughly the same colour temperature as daylight (5000-5600 K). When the ambient is a different colour temperature, such as tungsten (3000 K) or the greenish light from fluorescent lamps (4000K), you risk getting a weird and unnatural colours in your image image since there is no single white balance that can be chosen. In some situations you can correct this by using gel filters on the flash to make its colour match the ambient. But the simplest, and in some situations the only, solution to the problem of mixed lighting is to eliminate the ambient by making the flash the dominant light.
A second reason you might want the flash to be primary would be if you are shooting a subject that is moving quickly (athletes, dancers). You can eliminate motion blur by making the flash the dominant light since the flash will then freeze the motion because its duration is very short (usually only about 1/1000 second or less).
How to Make the Flash the Key Light
To make the flash the key light, you have to make the flash overpower the ambient light. This can be done with the flash in TTL mode by decreasing the exposure by three stops or more with shutter and/or ISO from the ambient setting you measured with the camera meter above.
Example: Assume the results of your metering above at ISO 200, f/4.0 indicated a shutter speed of 1/15 second. You would then recognise that you were in Fair ambient light.
To overpower the ambient light, we must reduce the impact of the ambient by at least -3 EV. We can do this by setting the shutter speed to 1/125 second (1/15 → 1/30 → 1/60 → 1/125). So by changing the settings and use ISO 200, f/4.0, 1/125 second, flash will be key and we will underexpose the ambient by three stops.
Notice that the highest shutter speed you can select when in regular flash sync is 1/250 second on a D200. This example is right on the edge of where you might have to turn off the flash and shoot available light or switch to TTL BL and go for balanced fill. In fact, if your camera is a D80 it has a maximum flash sync speed of 1/200, so you would not be able to fully overpower the ambient in this situation.
Example 2: Assume the results of your metering were ISO 400, 1/200, and f/4.0. You should immediately see that you will have major problems reducing the exposure of the ambient by three stops. You can't double the shutter, because you are already near maximum flash sync speed. That leaves only ISO and Aperture. You can reduce the ISO to 200 and that gets you one stop, but the only way to get the other two stops is to stop down the aperture by two stops (f/4.0 --> f/5.6 -- f/8). But if you change the aperture to f/8, the flash range will become less than six feet or so (using a diffuser), so that may not be an option. In fact, if this is your situation, you are forced to abandon the idea of overpowering the ambient light.
You must now consider balancing the flash to the ambient.
How to Balance the Flash to Fair Ambient Conditions
If you find yourself in a situation like Example 2) above, you could decide to turn off the flash and shoot available light or you could decide to balance the flash to the fair ambient.
There are two approaches to balancing the flash to fair ambient conditions:
- In TTL mode you reduce the flash compensation manually by trial and error until the right flash power is determined. I am not going to discuss this method.
- In TTL BL mode, the flash system will set itself automatically. This is the approach I will now discuss.
Recommended Initial Settings for allowing TTL BL to balance automatically with fair ambient: Camera P mode, Flash TTL BL, ISO 200. (notice that these are the same settings as for Bright Ambient conditions).
This will give you nicely balanced images with bright backgrounds and bright subjects. You might want to apply some negative flash compensation, say -0.3 ev, to keep it from looking like the subject is jumping out of the picture.
You might be tempted to use camera A mode, and it can be done, but you have to be very careful to prevent overexposure. The shutter is limited to 1/250 (or 1/200) flash sync speed, and since it can't go any higher than that, if you choose an aperture that is too wide, the shutter will bang into the limit and your images will all be overexposed. P mode fixes this problem.
Incidentally, among professional photographers the joke is that P mode stands for 'Professional' mode, because we all use it in bright ambient when shooting fill flash. For some reason, beginning photographers avoid P mode like the plague. P mode has its important uses, and shooting TTL BL is one of them (especially in bright ambient).
Quick Guide to Settings
- Use your camera's built-in light meter to determine the type of ambient light.
- Dark ambient light is the easiest situation to deal with. This is also a very common situation (indoors at parties, receptions, family life, etc.) Use the camera's Manual (M) exposure mode together with TTL or Auto flash for best results.
- Bright ambient light is a very common situation outdoors during the day. This situation calls for balanced fill flash. Let the camera's metering system control the background exposure, and use TTL BL or TTL to control flash exposure.
- Fair ambient light makes flash photography complicated. There is no “right” settings for this situation, and you need to weight your options carefully.
3. Basic Flash Use
Before I describe all the modes and features of Nikon's CLS in detail, and try to explain some of the theory of flash exposure, I'm going to present two quick and dirty recipes for basic flash use known as “bounce flash” and “fill flash”.
Mastering them requires little effort, and the theory behind the settings I've picked has been left out deliberately to keep things simple (but will be dealt with later in this note). Learning how and when to use just these two techniques will probably improve your flash photography in many situations. I hasten to add that these two techniques will not be usable in every possible situation were you need flash. But at least for me, between them they cover around 95 % of the situations were I use a single on-camera Speedlight.
“Bounce flash” is my preferred way of using a single on-camera Speedlight indoors in a room with normal height ceiling that is painted white or off-white. It is a simple technique to master, and usually works very well. Here is what you do:
- Mount the flash in the hot-shoe and select TTL (or Auto) mode.
- Tilt the flash head 45° upwards, so that the flash strikes your subject indirectly, via the ceiling. Make sure you tilt the flash sufficiently to prevent the subject or the background immediately behind the subject from receiving any portion of direct light from the flash.
- Zoom the flash zoom head to its widest setting. Some people also add a diffuser or other modifiers to spread the light even further, but I haven't seen any noticeable improvement in the quality of light when I do this.
- Set ISO 400.
- Select manual exposure control (
M) on the camera.
- Select a suitable shutter speed for flash (e.g. 1/60) and set the aperture to whatever you think will suit your subject.
- Pay attention to the monitor light on the flash. It signals that the flash has maxed out (a Nikon Speedlight do this by blinking for about 3 seconds), you may need to use a larger aperture or to increase the ISO. Re-shoot, review and if necessary open up the aperture or increase the ISO more. Repeat until you are happy with the result.
If the ceiling is too high, you may consider bouncing the flash off a white wall behind you or beside the subject. In many cases, using a wall will make more of the light reach the subject since the light does not have to travel as far.
When bouncing, the reflected light falling on the subject will be coloured by the colour of the surfaces from which it reflects. If these are not white or neutral grey, this may introduce a colour cast. As the colour temperature of the scene will be determined by the various surfaces in the room, great care must be taken with white balance. Turning the camera a few degrees may bring another surface into play which could influence the colour of the scene. Shoot RAW (NEF) so you can fine-tune the white balance in post-processing, and it is also recommended that you make use of a WhiBal or similar white balance card for setting accurate white balance.
Note: It is not always possible to use bounce flash. If there are surfaces that gives images an undesireable colour cast; or if the ceiling is too high or the room too large, you may not have access a large enough aperture or a high enough ISO to resolve step 8 in the procedure above. In such cases case, you may need to use direct flash.
Many people think of flash as a light source that may be used in an emergency, when the scene is too dark to otherwise take a picture. However, modern flashes are actually designed to be primarily used as a auxiliary or supplementary light source. That is, you may use flash to soften the shadows in a portrait taken outside at noon, with a bright sun overhead, or to lighten the foreground in a landscape at sunset.
This is known as “fill flash” and it is a very simple technique that works best in daylight out of doors. Here is a step-by-step recipe:
- Mount the flash in the hot-shoe and select TTL BL (or just TTL) mode .
- Point the flash straight ahead. Do not use a diffuser or other modifiers.
- If possible, use a D- or G-type lens.
- Make sure the camera's metering mode is set to matrix.
- Enable Auto FP so that you can use the full range of shutter speeds.
- Set ISO 200.
- Select aperture priority (A) on the camera.
- Set the aperture to whatever you think will suit your subject. Make sure the shutter speed picked by the camera is fast enough to avoid motion blur (if it is not, adjust aperture and/or ISO until it is).
- Review the shot. If you think the flash is too obvious and gives the photo an artificial look, dial in -1/3 EV flash output level compensation (FOLC). Re-shoot, review and if necessary add more negative FOLC. Repeat until you are happy with the result.
I usually end up using -2/3 EV for fill-flash. But you should experiment with different values until you find the “look” that suits you and your subject best.
Note 1: If you are using a Nikon Speedlight, make sure you use the TTL BL mode for fill flash. If you are using a dedicated flash that doesn't have a TTL BL mode, use TTL instead.
Note 2: It is not always possible to use fill flash. If the ambient light is too low, you may not have access a large enough aperture or a high enough ISO to resolve step 8 in the procedure above. In that case, you may need to use direct flash.
“Direct flash” flash is when the flash is positioned in the camera's hot-shoe or on a flash bracket directly above the camera, with the flash head pointing straight ahead along the same axis as the lens, when the flash is used as the main or sole light for the scene.
Direct flash does not flatter a subject, and should be avoided. It is, however, the type of on-camera that makes most of the light from the Speedlight. In some situations, direct flash may be the only way to get enough light on a dark subject to get a correct exposure.
Some people claim you can improve the quality of light by putting a light modifier in the shape of a diffuser dome or Gary Fong light-sphere on the Speedlight when using it for direct flash. This is nonsense. The quality of flash light is determined by surface. Bouncing improves the quality of light because the subject is lit by light reflected by the walls and the ceiling, which have a huge surface. Fitting a diffuser dome or a Gary Fong light-sphere on a Speedlight increases the surface area just a tiny amount, and in my experience it makes very little difference. These devices work by spreading the light wider. This makes sense only when there are white surfaces around that the light can bounce off. Bouncing without fitting such a contraption to the flash also seems to work equally well.
The bottom line is that direct flash is almost never nice. But sometimes you need all the power a single flash can deliver. Then, direct flash is your only option.
4. Modes set on the Camera
Some of the flash modes are set on the camera, rather than on the flash. To set most of these modes on a Nikon D80, you keep the flash button depressed and rotate the main (rear) command wheel to cycle through the modes. The modes appear in the flash frame in the top LCD panel on the camera.
To set Auto FP on a Nikon D80, you need to go into the custom setting menu (#25).
For other Nikon DSLR models, see the camera's instruction manual.
This mode is only available on cameras providing slow-sync flash.
The default shutter speed when using flash is 1/60 second (you can change this default on a D80 with custom setting #24). The flash will use a faster shutter speed if the ambient light requires it, but will not normally go below 1/60 second in the modes where camera controls the shutter speed.
When slow-sync flash is activated (on a D80, you set this by pressing the flash button and turning the main command wheel until the word “SLOW” appears on the camera's top LCD), shutter speeds as low as 30 seconds may be used to obtain the correct exposure for both the main subject in the foreground lit by the flash, and the background, in low-light situations or at night.
For long shutter speeds, you may need to use a tripod to avoid camera shake.
In the Night Portrait scene mode, slow-sync flash is automatically activated.
Red-eye Reduction Mode
This mode is only available with cameras having red-eye reduction control.
When this mode is activated (indicated by an eye-symbol appearing on both the camera's top LCD and the flash's rear LCD), the flash fires three flashes at reduced output just before the picture is taken. This makes the iris of humans and animals contract which reduces the area where the red eye effect can be seen. It also introduces a shutter delay to allow the subjects' irises time to react.
The red-effect only occurs when you use the flash as key light pointing forwards and directly into the subjects eyes. If you've read the section above about basic flash use, you know that you should avoid using your flash like this. I also think that the red-eye reduction mode is only moderately successful in removing red-eyes, and the shutter delay is annoying, so I don't use this mode.
But try it out! Perhaps you find more use for it than me.
This mode is only available with cameras providing rear-curtain sync.
Normally the flash fires at the same time the shutter curtain opens (i.e. front curtain sync). When rear-curtain is activated (indicated by the word “REAR” on the camera's top LCD), the flash fires just before the shutter curtain closes.
Rear-curtain sync may be selected when one is shooting fast-moving subjects at slow shutter speeds. With front curtain sync, unnatural-looking pictures can occur because the blurred movement appears to be in front of the subject frozen by the flash. Rear-curtain sync creates a picture in which the blur of a moving subject (for example, the taillights of a speeding car) appears behind the frozen subject.
Rear-curtain sync can not be combined with AWL, or with the FP or RPT modes.
FP (Focal Plane) is a sync mode that camera's normal maximum flash synchronisation shutter speed (often called “x-sync speed”), i.e., it lets you use flash at higher shutter speeds.
To use this mode on a Nikon DSLR, you need to enable Auto FP on the camera (custom setting #25 on the Nikon D80). Without having Auto FP enabled, the camera will not let you set the shutter speed to greater that 1/200 second (the x-sync speed of the D80) when you use flash. With Auto FP enabled, you can use any shutter speed.
In FP mode the flash will not fire once, but many times at an extremely rapid rate which begins with the opening of front curtain of your camera's focal plane shutter and ends with closing of rear curtain. This permits the correct exposure to be obtained as the travelling slit of a dual curtain shutter passes over the sensor at high-speed.
Note that using FP reduces the maximum power of the flash. The only way to have FP is to illuminate the focal plane curtain, which, depending on the shutter speed, may cover a very large percentage of the focal plane. This means that the higher the shutter speed is above the shutter's x-sync speed, the smaller the area of film that gets illuminated by each flash.
The table below shows approximately how much the maximum power expressed as guide number GN is reduced when an SB-900 is used in FP mode on a Nikon D80. It has full power (GN 34, ISO 100/meter) at the X-sync speed (1/200 second), is reduced by -1 EV (half power) at 1/250 second, and then by another -1 EV for each doubling of shutter speed. At 1/4000 second it is reduced by -5 EV, or 1/32 of full power.
Using FP will reduce the effective GN of the flash when you exceed the x-sync speed. However, it has no effect at speeds lower than the x-sync. It does no harm to have Auto FP permanently enabled.
On a Nikon Speedlight, the FP mode can be activated in the following flash modes: TTL, TTL BL, AA, M and GN. On the Nissin Di866 the FP mode only works in TTL mode.
Note: The FP mode is mainly useful when you want to use large apertures when doing fill flash in bright daylight. Because it makes you lose so much of the power of the flash, it is not very useful when you want to capture high speed movement. For high speed flash photography, you instead use manual mode, and adjust the power ratio to make sure that the flash burst is short enough to “freeze” movement. More more about this technique, see below.
High Speed Sync Without Auto FP mode
Not all Nikon DSLRs support FP mode. However, the following DSLR models: D70, D70s, D50 and D40 will sync with most generic flash units at any shutter speed (i.e. up to 1/4000 second). This is because these models use a CCD sensor that works as an electronic shutter at high shutter speed. A conventional mechanical shutter is only used at shutter speeds below 1/125 second.
For some reason, Nikon has put in a program limitation that will not let you set faster shutter speed than 1/500 second in these models if you mount a dedicated flash unit in the camera's hot-shoe. You can get around this limitation by taping over the two metal contacts on the back of the Speedlight.
5. Speedlight Flash Modes
Speedlights offer a number of different modes (not all Speedlights let you set all these modes). Here is a summary of the modes you may come across:
- TTL (Through The Lens): The flash output level is metered by a sensor in the camera had flash power is adjusted to expose the main subject is correctly, regardless of the background brightness.
- TTL BL (TTL BaLanced fill): Like TTL, but the flash output level is optimised for a well-balanced exposure of the main subject and the background.
- A (Auto): The light reflected by the scene is metered by a sensor inside the flash itself and this measure used to determine flash power level.
- AA (Auto Aperture): The same as A, but automatically adjusts the aperture setting on the flash to match the aperture on the camera.
- M (Manual): The flash power level is set explicitly by the user.
- GN (Distance Priority Manual): The power level is derived from the distance to main subject, as set by he user.
- RPT (Repeating): Like M, but the flash fires repeatedly during a single exposure.
How these modes operate are described in more detail below:
When the flash is used in TTL mode, a low-power pre-flash is fired prior to exposure and before the shutter opens. The light generated by this pre-flash is measured by a sensor inside the camera (i.e. Through The Lens). A computer inside the camera then computes what power from the flash is required for correct exposure. Then the shutter opens, the exposure is made, and the shutter closes.
All this happens so fast that you'll not be able to tell the flash apart from the pre-flash (but unfortunately not fast enough to stop people with fast reflexes from blinking in response to the pre-flash).
Nikon Speedlights let the photographer choose between two different TTL modes: A plain TTL mode, and the TTL BaLanced fill (TTL BL) mode.
In the plain TTL mode, the camera's standard exposure
meter measures the ambient light using the light measurement mode
(matrix, centre weighted or spot) selected. It uses this measurement
to select the exposure settings for the camera, such as aperture and
shutter speed. It will do this without taking into consideration that
flash will be used. Also, the camera will make a separate
centre weighted measurement of the reflected light from the pre-flash.
Note that TTL and TTL BL metering is not influenced by the light measurement mode selected for the camera. It is always heavily biased towards the centre (it does not follow the focus point). If your main subject is off-centre, use FV lock to lock flash exposure while having your main subject in the centre, then recompose.
In plain TTL mode, the two measurements will not be interfere with each other. The flash output power, and the shutter speed and/or aperture setting (depending on exposure mode), will be decided by separate computations.
If you want the subject should stand out strongly from the background, you should use TTL mode. In this mode, the system expects that the flash will produce most of the light and that the subject should be clearly illuminated.
TTL is also the most appropriate mode for macro work and for interiors when the flash is the dominant light source and for bounce flash. Also, when doing a portrait shot outdoors at night or in a large, dark room, TTL will probably give you better exposure than TTL BL.
As described above, when you are using the flash in plain TTL mode, the flash metering system does not take the ambient into account when determining how much light to put out. However, the ambient will add to the exposure. If the ambient is not dark, the added light from the flash can overexpose the subject. So, if the ambient light is fair or bright, and you want to use the plain TTL mode, you should reduce the flash power by setting some negative FOLC to avoid overexposure. The amount to use is a judgement call, which is one of the things that make using TTL flash in difficult in fair and bright light.
The TTL BaLanced fill (TTL BL) mode is the most complicated flash mode on the Nikon Speedlights. This mode is (so far) not found on any third party flash units.
Originally, BL meant “BackLit” which was a strong hint about what type of situation Nikon's engineers had in mind when they designed this mode. At some point, the Nikon's marketing department decided that “BaLanced fill” had a better ring to it. The original name, however, describes better the type of situation this mode is best suited for.
As the original name suggested, the TTL BL mode is designed to be used for fill flash when the main subject is back-lit (i.e. the background is brighter than the subject). An example of a back-lit scene is an outdoor portrait where the sun is behind the person, or a scene where the person is photographed with the bright sky as background. Another typical back-lit scene would be indoors during daytime, when you position the subject in front of a window that includes the much brighter outdoors in the frame.
The TTL BL mode starts out, like TTL node, by making two separate exposure measurements prior to the exposure: One is a centre-weighted measurement of the pre-flash reflected by the main subject, and one of the ambient light lighting up the bright background using the light measurement mode (matrix or centre weighted) selected for the camera.
However, it differs from the TTL mode by combining the measurement of the reflected pre-flash and the ambient. The intent is to outout just enough power from the flash to make the flash light falling on the main subject match or balance the ambient light falling on the background.
To be able to use the flash in TTL BL mode, you must use 3D matrix metering (recommended) or centre-weighted metering. Spot metering will force the camera to TTL (TTL BL only makes sense if the camera is aware of the ambient light and the brightness of the background.)
For the most accurate results, the use of Nikon D- or G-type lenses is also recommended. The distance information from the lens and 3D matrix metering is then used in the computation to determine the amount of flash light to put out.
The flash power calculation is also based on centre-weighted metering of the light reflected by the pre-flash. If your main subject is off-centre, you should use FV lock to lock flash exposure while having your main subject in the centre, then recompose.
The calculations involved in balancing the light in TTL BL mode are complex. In certain circumstances under- or overexposure may occur. To help you anticipate these circumstances, some of the situations where the TTL BL mode may not work well, are outlined below:
- If the main subject is better lit by the ambient light than the background, with no shadows to fill in, there is no point in using the TTL BL mode.
- If the background is very dark (e.g. a stage with black walls) or very light or reflective (e.g.: an ice hockey match), then the complex automatic measurement and control system required to may be fooled into under- or overexposure.
- If the composition have the main subject only occupying a small part of the frame compared to the overall field of view, the flash calculation may be based upon the background. This may result in an erroneous flash power setting for getting a correct exposure of the main subject.
- When using a wide angle lens, dominant light sources or sinks (dark regions which will not reflect light) may affect the flash calculation and may result in erroneous exposure.
- If the camera is being used for longer times at a high frame rate the power drain on the batteries in the flash system may become too great to support the sustained highly critical measurement that TTL BL fill flash requires. In this case one shot out of a sequence may be affected as the equipment attempts to cope with falling flash battery voltage and high current demand.
In such situations, you will probably get better results if you use the plain TTL or non-TTL Auto mode. In some cases, you may need to take full control and switch to Manual mode.
The non-TTL Auto (A) and Auto Aperture modes found on some Nikon Speedlight and the Aperture value (Av) mode on the Nissin Di866 are modes for automatic flash exposure control that operates like the classic “auto-thyristor” flashes from the 1970ies (e.g. the the Vivitar 283).
In the auto modes, instead of measuring through the lens, a built-in sensor at the front of the flash measures the average flash light reflected by the entire scene and use this to determine the output level of the flash. The measurement is usually done in real time during the actual exposure, and the flash is turned off by a thyristor circuit within the flash when the sensor has determined that the scene is sufficiently lit.
The auto modes are not available on the SB-600. On the Nikon SB-800, there is no pre-flash in the Auto mode, but a pre-flash is used in Auto Aperture mode. On the Nikon SB-900, you can toggle pre-flash on and off in both auto modes. On the Nissin Di866 Aperture value mode) there is no pre-flash and you have to enter both aperture and ISO “by hand”.
On Nikon Speedlights, ISO and focal length are communicated from body to flash in both Auto modes. The Auto Aperture also communicates the aperture, while in the the plain Auto mode, you must set the aperture on the flash “by hand”. This emulates the way the original “auto-thyristor” flash units worked, and means that you can quickly dial in any amount of FOLC by “lying” to the flash about what aperture you use. On the Nissin Di866, this mode is called Aperture value (Av) and you have to set both aperture and ISO “by hand”.
The pre-flash sequence that is intrinsic to the TTL modes introduces a tiny shutter delay. Since the auto modes let you control the flash in real time, you can use the auto modes without a pre-flash and therefore without this shutter delay.
When you set one of the manual modes, nothing measures the light. You control the power of the flash by setting the desired power ratio (M) and (RPT), or distance to the subject (GN), on the flash.
In vari-power Manual (M) mode, you control the output power of the flash by setting the desired power fraction from “1/1” to as low as “1/128”. Every time you half the power setting, power is reduced by -1 EV (i.e. one f-stop). Most flash units let you adjust the power steps equal to 1/3 EV.
The power of the flash is actually determined by the duration of the burst of flash light. The less power you set, the shorter the duration is. The table below show the duration of flash as fraction of a second for four different flash units.
This feature of manual flash is very useful for high speed flash photography. This is a technique very you photograph fast moving objects (e.g. drops splashing into water) and use short-duration flash to “freeze” movement. To minimise shutter delay, it is common to open the shutter before the action starts, and instead make the flash determine the instant of exposure. For this to work, you need the room to be so dark that the ambient does not impact on exposure. For much more about this type of photography, see hiviz.com.
Repeating flash (RPT) is a special manual mode where the flash fires repeatedly during a single exposure. This may be useful for a technique known as stroboscopic motion photography, where stroboscopic (i.e. pulsing or repeating) light is used to capture multiple images of certain fast-moving bright objects set against a dark background on a single frame (e.g. a bouncing golf ball).
When using repeating flash, you should use the following formula to work out the shutter speed:
Shutter speed = Number of strobes / Frequency
For example, of you set up the Speedlight to fire 20 strobes at 10 Hz, the shutter speed should be set to 20/10 = 2 seconds.
You will lose power in the RPT mode. There is a table in the flash's manual that will tell you how much you lose, depending upon the strobe frequency and the number of strobes you want to fire. For example, a SB-900 Speedlight set up to fire 6 strobes at 6 HZ, will fire at 1/8 of full power. With 24 strobes at 100 HZ, the flash will fire at 1/128 of full power. This means that with a single Speedlight, repeating flash is only feasible to capture small objects at close range. You will need an industrial strobe for a full body shot (or a whole bunch of Speedlights firing in sequence).
Traditionally, repeating flash made it possible to make multiple exposures at higher frame rates than continuous shooting with a DSLR. However, the continuous shooting modes of some DSLRs offer much higher frame rates than the motor-drives of the film era, and high-speed video is also becoming more and more available. These, and other advances in high speed photography, is making repeating flash less important as a means of capturing fast movement.
Finally, the distance priority manual mode (GN) is a mode where the flash controls the light output by taking the distance you set at the back panel and translating that distance to a power setting by means of the flash's guide number. If you don't understand what that means, don't worry. It is not a mode most people will find very useful. Only Nikon's top-of-the-line Speedlights offer this mode.
6. Understanding Flash Exposure
There is nothing that stops you from using flash in one of your camera's fully automatic exposure programs (green Auto, or one of the vari-programs). If you do, the camera will take care of setting the shutter speed, aperture and (for the D300s) ISO for you (for other Nikon DSLRs, auto ISO is always disabled when a dedicated flash is in the hot-shoe). If you also use one of the automatic modes on the flash, you will not have to deal with setting exposure yourself. All you have to do is to frame and focus.
Fully automatic exposure settings do not give you much control over things. To pick these settings yourself, you need to understand how your Speedlight interacts with the camera's exposure settings (shutter speed, aperture and ISO) and also how the light from the flash interacts with the ambient light.
How all this come together is explained below:
Flash and Shutter Speed
If we consider a high-contrast scene were some parts are lit by bright sunlight, and other parts are in deep shadow, we'll find that while the sun will outshine the flash in the sunny spots, the light from the flash will have the most impact in the shadows. Also, the output of a camera-mounted Speedlight decreases in proportion to the square of the distance. In other words, each doubling of distance will reduce the output to 1/4 (1/22).
This means that the flash will have the greatest impact on objects that are in the shadows and on objects that are near the camera. Objects that are already brightly lit by the ambient and objects that are further back (in the background) will receive much less impact from the flash and a greater proportion of their exposure will be caused by the ambient light.
Aperture and ISO works the same way with flash as they do without. Shutter speed, however, works somewhat different, so in the rest of this section, we shall focus on shutter speed.
The duration of the flash blink is much shorter than any shutter speed you can use. As a result, the shutter speed does not influence the impact of the flash upon the exposure. Instead the shutter speed determines the exposure for the parts of the scene were the flash light has the least impact (i.e. the highlights and the background).
The fact that the shutter speed does not impact on the flash's contribution to the exposure is the basis for a very useful technique known as “fill flash”. The main idea behind this is to set a combination of shutter speed and flash output power where the light contributed by the flash only make a auxiliary impact on the scene, filling in the shadows, but otherwise letting the scene be lit by ambient light.
Unless the camera is used in Manual (M) or Shutter priority (S) mode (where you pick the shutter speed), the camera's built in light meter will try to set a shutter time that will give you correct exposure for the ambient light.
However, when the camera senses a dedicated Speedlight in the hot-shoe, by default, the maximum shutter speed in the P and A exposure modes are restricted to 1/60 second or shorter for “safe” handheld operation (you can change this limit in the camera's custom settings if you think that your hands are more steady than Nikon's engineers). You can override this default by dialling in Slow-sync Flash.
Likewise, the maximum shutter speed that in the P and A exposure modes are restricted to the x-sync speed or longer. You can override this by setting Auto FP.
By setting the shutter speed long enough, you may be able to capture enough of the ambient light to get the exposure right for fill flash. However, this may result in shutter times unsuitable for moving subjects, or may make it necessary to use a tripod.
Fill Flash or Key?
In many situations, you might wonder whether it is possible to use fill flash (where the flash light is used as auxiliary light to supplement the ambient light), or you have to use flash key (where the flash is your main or key light). If you are filling in the shadows on a sunlit day, fill flash is probably what you should use. But if you are working indoors where the ambient light may be much lower, you need to evaluate the quality of the ambient light before making a decision.
Provided that your flash is powerful enough, you should always be able to use your Speedlight for as your key light. However, to use it for fill flash in a dim or dark setting is not always be an option.
To check if you can use your Speedlight for fill flash, do the following:
The exposure of the background will mainly depend on your shutter speed. Select the slowest shutter speed you can use without getting motion blur with the focal length in use. To make sure that the shutter speed will give adequate exposure, check the electronic analog exposure display at the bottom of the viewfinder. If this indicates an underexposure of more than 3 EV, as shown (green) in the example above, you will not be able to use fill flash. In that case, you need to open up the aperture more and/or increase the ISO. If the ambient light is low enough, you may run out of f-stops or usable ISO values without ever getting adequate exposure. In that case, give up trying for fill-flash and use flash key instead.
If the analogue exposure display tells you that you are within the 3 EV range, you should be able to use the TTL BL mode (if available) or TTL mode for fill flash.
If you are unsure about how flash light and ambient light mixes, you can check the result by using the LCD screen on the camera for a review and to look at the histogram. (If you're unfamiliar with histograms, see this note by Ron Day: Interpreting & Using Histograms.)
If the histogram and/or review image on the camera's LCD screen reveal a problem, such as clipped highlights or blocked shadows, you may correct the problem by using exposure compensation.
You may do this on the camera and flash. The former is referred to as EV compensation (EV) and the latter is known as flash output level compensation (FOLC). Use EV on the camera to modify the exposure of the background, and FOLC on the flash to modify the exposure of the main subject.
Note that if your get underexposure because your flash is underpowered for the task at hand (e.g. your main subject is too far away, or you are trying to light the background on a huge, dark set with a single Speedlight), setting a positive FOLC will not have any effect. You instead need to bring a more powerful flash unit, open up the aperture, or increase the ISO.
To lighten the background only, set the camera's exposure mode to S (shutter priority) or M (manual), and set the shutter speed to a long enough shutter speed to give he background the right exposure without flash. You may have to set the camera's flash sync mode to slow sync and use a tripod to bring out background details in some low-light situations.
7. Case Studies
So much for the theory. Let us move on and look at some typical situations where you can use these exposure modes for a particular effect.
Case #1: Balancing Ambient and Flash Light (TTL BL)
Flash is not just for use indoors and in dark conditions. As many photographers know, portraits taken in bright outside sunlight may sometimes produce a difference in a tonal range between skin lying in shadow and skin areas exposed to the full sun that is well beyond the dynamic range of any film or sensor. In addition to direct sunlight, strong overhead spotlights, highly directed daylight from a window, and other light sources where the light falls on the subject from one direction only may create problems. Not only is the resulting large shadows problematic and may exceed the dynamic range of the camera, but also the small shadows, particularly in portrait work, may accentuate wrinkles and distort features.
In this type of lighting conditions, a gold or silver reflector disc close to the subject is an excellent mechanical means of evening out the illumination, but a reflector disc is not always available.
However, by adding the right amount of light from a flash to such a scene, we can come home with an image that retains the feeling of the ambient light and at the same time compacts the dynamic range down to something that the photographic process can handle.
In these cases, a single camera mounted Speedlight set to TTL BL mode may save the shot. In this mode, the camera measures both the available light falling on the subject (taking into account the focus distance with lenses that transmit this information) and also that of the surrounding (background) area of the image. In addition pre-flashes are emitted from the Speedlight to measure the effect of flash light on the scene. When the shutter curtain lifts, the flash fires with a power output based on a computation combining the ambient light metering and the pre-flash measurement, so that the flash just “fills-in” the foreground.
I never use the green Auto exposure mode or any of vari-programs in combination with the TTL BL mode on the flash. These programs give very little control to the photographer. However, all the camera's conventional exposure modes (P, A, S and M) works fine with TTL BL mode, but I find the aperture priority (A) to be the most convenient. It gives me control over the aperture so that I can fine-tune the depth of field, while the camera takes care of the task of finding a shutter time that results in a nice exposure based upon the ambient light.
Case #2: Reducing the Effect of Mixed Lighting (TTL)
Our eyes automatically adapts to the colour of the light that illuminates a scene. A white wall appears white to us when it is lit by daylight, by tungsten, or by fluorescent light. But this is not how the camera see light. Daylight has a colour temperature between 5000 and 5600 K). tungsten has a colour temperature around 3000 K, fluorescent light has a colour temperature around 4000 K, and to the camera, these different colour temperatures may cause colour casts on the entire scene unless we compensate.
Film photographers did this by using a film type (e.g. daylight film, tungsten film) that matched the colour temperature of the light illuminating a particular scene. Digital cameras let you explicitly set the the colour temperature. They also have an automatic setting where the camera does this for you. As long as all the light illuminating a scene have the same colour temperature, you should be able to avoid colour casts.
The problem arise when there is more than one source of light. Nikon Speedlights have roughly the same colour temperature as daylight. When the ambient is a different colour temperature, such as the reddish light of tungsten or the greenish light from fluorescent lamps, you risk getting unnatural colour casts in your photograph. For example, daylight from a window or strong overhead fluorescent lighting would all have a different colour temperature and could cause different colour casts on the different faces within a group shot, or even across the face of a portrait.
In some situations you can correct this by using gel filters on the flash to make its colour temperature match the ambient. But the simplest, and in some situations the only, solution to the problem of mixed lighting is to eliminate the ambient by making the flash the dominant light.
When you want to make the flash dominant you must reduce the impact of the ambient. You do this by picking the highest possible shutter speed that allows syncing with the flash at full power (on most Nikon DSLRs, this is 1/200 of a second) to minimise the impact of the ambient light. By having the flash imn TTL mode, exposure will be determined by the how much power the flash put out.
Case #3: Eliminating shutter delay (Auto)
Both Nikon's TTL modes use a pre-flash sequence that introduces a tiny shutter delay. For the fastest response (e.g. skateboard photography) you may want to use a flash mode that provides you you automatic flash power control, while at the same time eliminates the shutter delay caused by the pre-flash sequence.
There is no need for a pre-flash in the Auto-mode. Without pre-flash, the delay when you press the shutter button is shorter. The difference is small, but it may make a difference in responsiveness when you shoot sports with flash.
Note: If you use the Auto Aperture (AA) mode on the Nikon SB-800, it will – for some strange reason only known to Nikon – emit a pre-flash. If avoiding pre-flash is your reason for selecting this mode, make sure the mode you select is Auto (A). You toggle between AA and A in the flash setup menu. On the SB-900, you can toggle off pre-flash on and off in both the AA and A modes.
Case #4: Working in Controlled Conditions (Manual)
Most of the time, you will probably rely on one of the TTL or Auto modes to calculate the right amount of flash power for each individual shot.
However, when you are working in a studio or some other strictly controlled environment, it may preferable to have full manual control over every parameter – shutter, aperture, ISO and flash power. In that type of situation, you set your Speedlight to Manual (M). You then determine what power ratio to use for correct exposure, either by means of a hand-held flash meter, or by using your DSLRs review image and histogram to judge exposure.
When you use manual settings, you don't have to second-guess what the system is going to do next. You can rearrange props and swap between light or dark backgrounds without having to check and counteract any automatic variation in flash strength. When you've set up controlled conditions, you don't want your Speedlight to make up its own mind up about the exposure.XXXXXXXXXXXXXXXX
A second reason you might want the flash to be primary would be if you are shooting a subject that is moving quickly (atlethes, dancers). You can eliminate motion blur by making the flash the dominant light since the flash will then freeze the motion because its duration is very short (usually only about 1/1000 second or less).
Case #5: Shallow DOF in bright light (FP)
There are certain circumstances where you may want to use flash to soften the shadows, and at the same time you may want to throw the background out of focus using a large aperture. However, a high level of ambient lighting means that you will need to use a shutter speed faster than the camera's x-sync speed (1/200 second on most Nikon DSLRs) to get the aperture you want. In this case a special mode called FP can be used. It lets you set any shutter speed your camera can muster.
8. Usage Notes
Dealing with “blinking”
Some people has fast enough reflexes to blink when they are exposed to the pre-flash used to compute exposure in TTL mode (and other modes that uses pre-flash). Some possible remedies are listed below:
Method 1: You can avoid blinking by using FV (flash value) lock. Use the FV-button on the camera to fire the pre-flash so long ahead of the flash that any blinking has ceased when you take the actual image.
You activate the FV lock by pushing a button on the camera. Only Nikon's higher end DSLRs (D200 and better) comes with a dedicated FV lock button. Some lesser models let you use custom functions to reassign buttons. For instance, on the D80, you can reassign FV lock to the generic Func-button.
Method 2: The 0.4 second shutter delay that you can set on some Nikon DSLRs to allow mirror-slap induced vibrations to dampen before the shutter opens also work well on most “blinkers”. On bodies without a FV lock function, I often use this delay instead of method 1.
Method 3: Some flash modes does not use a pre-flash. This is the case with Manual (M) mode on all flashes, the Aperture value (Av) mode on the Di866, and the Auto mode on the SB-800. On the SB-900, you can turn off the pre-flash in both the Auto (A) and Auto Aperture (AA) modes. Using a mode without pre-flash gets around the blinking caused by pre-flash.
Non-CPU Lenses and Flash
If you use a lens without CPU, like an old Nikkor AI-S lens, you can not use any of the TTL-modes mode, but the modes where there are no pre-flash, or where you can toggle pre-flash off (e.g. Auto and Manual) will work fine.
9. Final Words
The first instalment of this series dealt with CLS terminology and features.
In the next instalment if this series (forthcoming), I will discuss Nikon's Advanced Wireless System (AWL) and other means of using wireless off-camera operation of multiple flash units.