Nikon Flash Use

1. Introduction

Nikon DSLR's and dedicated flash units has very sophisticated control with a number of modes and settings. Some are set on the camera. Others are set on the flash. This note has put a description of all of them in one place, along with a set of case studies giving real-life examples of use.

2. 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 setting flash moes on camera or other Nikon DSLR models, see the camera's instruction manual.

Slow-sync Flash

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 subject's 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. Later on in this series. when I discuss direct flash use, I'll tell you 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.

Rear-curtain Sync

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.

Auto FP

FP (Focal Plane) is a sync mode that let you synchronise flash with higher shutter speeds than the camera's normal maximum flash synchronisation shutter speed (often called “x-sync speed”).

To use this mode on a Nikon DSLR, you need to have a CLS-compatible flash unit connected to the camera's hot-shoe, and you must 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 higher than the x-sync speed when you use a CLS-compatible 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 (typically 50 KHz) 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.

Shutter Speed	GN	Power
1/200	34	0 EV	1/1
1/250	22	-1 EV	1/2
1/500	16	-2 EV	1/4
1/1000	11	-3 EV	1/8
1/2000	8	-4 EV	1/16
1/4000	6	-5 EV	1/32

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. The FP mode does not help you freeze motion. Normal flash photography is very good at freezing motion, since a burst of electronic flash is so incredibly brief. When the dominant light on a scene is a very short flash of light it is almost as if you used a very high shutter speed in the thousandths of a second. However when you use FP mode flash, the flash unit pulses the light output over a longer period of time in order to simulate a light being lit for the entire time the shutter travels across the focal plane. Since the flash burst is no longer very short, it becomes more difficult to freeze motion, even with high shutter speeds. And because the FP mode makes you lose so much of the power of the flash, it is not very useful when you want the flash to be the dominant light. 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. For 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.

3. 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
  • TTL (Through The Lens): The light from a pre-flash reflected by the centre subject is metered by a sensor in the camera and flash power is adjusted accordingly to expose the centre subject is correctly.
  • TTL BL (TTL BaLanced fill): Like TTL, but the algorithm used by the in-camera computer to figure out the flash power setting is more complex and takes additional data into consideration.
• Auto
  • 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.
• Manual
  • 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:

TTL

When the flash is used in TTL or TTL BL 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 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. In TTL mode, this measurement alone is used to compute the power to use when the flash is fired.

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. Also in plain TTL mode. distance data, as reported by G- and D-lenses, is not taken into account when computing flash power.

Note that in TTL mode, metering of the pre-flash 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.

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 may 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.

TTL BL

The TTL BaLanced fill (TTL BL) mode is the most complicated and sophisticated flash mode on the Nikon Speedlights. If you try to read about TTL BL on books and Internet sites, you fill find a lot of different descriptions. Desmond Downs argues that most of these descriptions are outdated. I think he is right. My description below has been updated after I read Downs' blog and reproduced his tests. The description of TTL BL that follows is based on extensive experiments with a Nikon D80 and a Nikon D700, together with a Nikon SB-900 flash, in an attempt to determine what really goes on in the TTL BL mode. The findings are believed to be correct for the Nikon D200 and newer bodies. Older Nikon DSLRs (i.e. D1, D1H, D1X, D2H, D2Hs, D2X, D2Xs and D100) used a separate, secondary metering system for TTL flash exposure that worked differently. This is now mainly of historical interest and will not be described here.

The TTL BL mode starts out by making two separate exposure measurements prior to the exposure. One is a focus-point and highlight biased measurement of the pre-flash reflected by the main subject, and one of the ambient light lighting up the subject using the light measurement mode (matrix or centre weighted) selected for the camera.

Since the TTL BL flash measurement is biased towards the active focus point, there is no need to use the FV lock when you're using the TTL BL mode, even if your subject is off-centre. Just make sure your subject is in-focus.

In the TTL BL mode the camera computer may take the distance to the in-focus subject (as reported by a G- or D-type lens lens) into account when computing flash power. (However, it only does if the flash is not bounced and the focus distance is within flash range. If the flash is bounced or focus distance is outside the flash range, it does not use the distance data.) This means that if your focus is off, the camera may use the wrong distance data and you will end up with a picture where your main subject is out of focus and under- or overexposed because the flash fired with the wrong power setting.

Further, in TTL BL mode the camera's exposure computer tries to combine the measurement of the reflected pre-flash and the ambient. The intent is to output just enough power from the flash to make the subject adequately lit. The flash power algorithm seems to be exposing for the highlights. It seems much less inclined to overexpose highlights than the algorithm used in TTL mode. I seldom find the need to use negative FOLC in TTL BL mode.

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.

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 computations involved in determining flash power 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 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.

Auto

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 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.

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.

Manual

Nikon offer three different manual modes on some of its Speedlights: A vari-power Manual (M) mode, stroboscopic repeating flash mode (RPT), and something called distance priority mode (GN).

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 in 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 shows the duration of flash as a fraction of a second for four different flash units.

Power		SB-600	SB-800	SB-900	Di866
1/1	0 EV	1/900	1/1050	1/880	1/600
1/2	-1 EV	1/1600	1/1100	1/1100	1/900
1/4	-2 EV	1/3400	1/2700	1/2550	1/1500
1/8	-3 EV	1/6600	1/5900	1/5000	1/3200
1/16	-4 EV	1/11100	1/10900	1/10000	1/5000
1/32	-5 EV	1/20000	1/17800	1/20000	1/9000
1/64	-6 EV	1/25000	1/32300	1/35700	1/15000
1/128	-7 EV	-	1/41600	1/38500	1/22000

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.

4. 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: 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 in TTL mode, exposure will be determined by the how much power the flash put out.

Case #2: 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, I find programmed auto (P) to be the most convenient. In the programmed auto mode, in suitable light, the camera will give you sensible settings for shutter speed and aperture that balance the flash and the ambient. If you want to fine tune, say, the depth of field, the programmed auto mode will let you do that, and automatically adjust the shutter to give you the correct exposure based upon the ambient light.

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.

Case #5: Shallow DOF in bright light (Auto 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, if we try this in bright ambient we may find that the aperture we want is only possible if we set a shutter speed faster than the camera's x-sync speed (1/200 second on most Nikon DSLRs). In this case a special mode called Auto FP can be used. It lets you set any shutter speed your camera can muster.

5. 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 and aperture for you. 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.

Exposure compensation

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.

ISO

How auto ISO operates with flash depends on the camera model.

In the Nikon D300s (and later?), if you have auto ISO enabled, the camera will set the ISO based upon the ambient without taking into account that flash will be used.

On some older models, having auto ISO enabled would increase the ISO if the the pre-flash measurement indicated that the built-in or hot-shoe mounted flash would be underpowered at the base ISO setting, without taking the ambient into account. In these models, auto ISO would be disabled when you use AWL to control off-camera Speedlights.

To determine how your camera model works when auto ISO is used with flash, with and without AWL, you may want to experiment.

6. Read More

The article you are reading is part of a series about dedicated flash for Nikon cameras. Here is an overview of the series:

• Introduction to Nikon's CLS – terminology and features.
• Overview of Nikon Speedlight models compatible with the CLS.
• Basic settings for different ambient lighting situations.
• More settings and modes, both on the camera and on the flash (this article).
• Flash receipes for using a single on-camera Speedlight.