Nikon Flash Use
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.
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 the flash modes on camera or other Nikon DSLR models, see the camera's instruction manual.
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 slow 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.
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. The difference is illustrated with the two images below.
Rear curtain sync automatically enables slow-sync mode, since the only time you should want to use rear sync, is when you also set a slow shutter speed to capture blurred movement.
Using rear curtain sync with shutter speeds faster than 1/30 second is not recommended. Rear curtain sync introduces a delay between the pre-flash and the main flash lasting the length of the exposure. This delay gives most people just enough time to react to the pre-flash. Using rear sync for portraits is a sure way to make your subjects blink or squint.
Rear curtain sync can not be combined with AWL, or with the FP or RPT modes.
Red-eye Reduction Mode
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.
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.
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 (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: 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.
- 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 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).
In some situations, the photographer may have a choice between two different TTL modes:
If you're shooting with a Nikon Speedlight SB-600, SB-800 or SB-900 connected to the hot-shoe, there is two slightly different TTL-modes to choose from: Plain TTL mode, and TTL BL mode. You should pick the plain TTL mode if you use spot metering, and TTL BL mode if you are using matrix or centre weighted metering. The Nikon Speedlights SB-400 and SB-700 automatically sets the TTL mode that best matches the light metering mode set on the camera. If you're using advanced wireless flash (AWL) in TTL mode, the system will always use TTL BL.
the plain TTL mode, the camera's standard exposure meter
measures the ambient light using the light meter mode selected on the camera.
The camera uses this measurement to select
the exposure settings for the camera, such as aperture and shutter
speed in the non-manual exposure modes. 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 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.
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 for some reason do not want to use the TTL BL 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.
I consider the plain TTL mode to be a legacy mode that has now been replaced by the much better TTL BL mode as Nikon's default flash mode, and tend not to use it much.
The TTL BL mode is now Nikon's default TTL mode when the camera is set to matrix or centre weigthted metering. On the Speedlight SB-900, SB-800 and SB-600 you can still force plain TTL when the flash is mounted on the camera, but as soon as you move the flash off-camera, it will automatically operate in TTL BL mode unless you select spot metering. If you are using the Nikon SB-400, SB-700, or some third party dedicated flash, you do not have the option to use plain TTL if the camera is set to matrix or centre weigthted metering. Instead, the TTL BL mode is selected automatically when the camera is set to matrix or centre weigthted metering, and plain TTL mode is selected automatically when the camera is set to spot metering.
The TTL BL mode is the most complicated and sophisticated flash mode on the Nikon Speedlights. If you try to read about the TTL BL mode in books and on 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 revised after I read Downs' blog and reproduced his tests. In these outdated descriptions, you may see that the author refers to the TTL BL mode TTL BaLanced fill or TTL BackLit, and the idea that TTL BL should be restricted to fill flash and/or backlit subjects. This is no longer the case. The re-engineered TTL BL is designed to be used both for flash key and flash fill.
The description of TTL BL that follows is based on extensive experiments with both 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 BL flash exposure that worked differently. This old version of TTL BL 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 tied to your main focus point.
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 main 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 highly reflective surfaces may affect the flash calculation and result in underexposure. Sinks (dark regions that do not reflect any light) may for the same reason result in overexposure.
- 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 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 or SB-700. 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. Since there is no pre-flash, the Auto mode is also handy when photographing people that blink when exposed to the pre-flash.
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 five 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. For typical drop splashes, a flash duration of 1/20000 is suitable.
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 stroboscope 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. 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.
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 Exposure Value (EV) adjustment. The latter is known as Flash Output Level Compensation (FOLC). Use EV on the camera to modify the exposure of both the background and the output of the flash, and FOLC on the flash to modify the flash output level only.
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.
How auto ISO operates with flash depends on the camera model.
In all Nikon DSLEs produced since 2009 (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.
5. 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.