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Using a Single CLS Speedlight

by Gisle Hannemyr

Table of Contents

  1. Introduction
  2. Exposure
  3. Case Studies
  4. Pointers

1. Introduction

In 2003, Nikon launched a new, sophisticated flash control system, known by the abbreviation CLS (Creative Lighting System).

For an introduction to CLS, see the first installment in this series of notes on using CLS.

CLS provides the photographer with a number of new options in the field of flash photography with digital cameras. It is very powerful and flexible system, but the number of features and options can be confusing at first. This note is a short tutorial on how one can make use of the some of CLS features with a single Speedlight in the camera's hot-shoe. (A forthcoming note will discuss wireless off-camera operation of multiple Speedlights.)

Disclaimer: Unfortunately, Nikon's documentation does not go into much detail about CLS, so the findings below is based upon my limited experiences with the system on equipment I have used (i.e. Nikon D80 body and Speedlight SB-800). The information may be incomplete and inaccurate. Additions and corrections are welcome.

2. Exposure

Nikon Speedlights offer a number of different modes. Here is a summary:

When you use the TTL BL or TTL flash modes, a low-power pre-flash is fired prior to exposure. The correct power to use for flash during exposure is determined by measuring the amount of the pre-flash reflected by the entire scene or foreground subject through the camera lens (TTL = Through The Lens).

When you use AA (Auto Aperture) flash mode, a low-power pre-flash is also fired prior to exposure, but 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.

When you use A (Auto) flash mode, you use also built-in sensor at the front of the flash for metering, but in this mode the measurement is done in real time during the actual exposure, and the flash is turned off when the sensor has determined that the scene is suffieciently lit. You cannot use FV lock in this mode, since there is no pre-flash to lock on.

When you use a M (Manual) flash mode, nothing measures the light. You control the flash's contribution to the exposure by setting the desired power ratio on the flash. To get the correct exposure, you also need to manually set the appropriate ISO, aperture and shutter time on the camera.

FP (Focal Plane) is a special high-speed sync mode that you may use with some of the other modes. It lets you exceed the camera's normal maximum flash synchronisation shutter speed (often called “x-sync speed”).

Setting 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 (and sometimes also ISO) 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. Afterwards you should check that the exposure in fact is OK 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 reveals problems with the exposure, such as clipped highlights or blocked shadows, you may correct the problem by using the right amount of exposure compensation.

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.

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.

The output of a camera-mounted Speedlight decreases in proportion to the square of the distance. In other words, each dobling of distance will reduce the output to 1/4 (1/22). This means that the objects near the camera will receieve a lot of light from the flash, while objects further back (the background) will receive much less light from the flash and instead receive a larger proportion of their light from the surroundings.

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 inpact of the flash upon the exposure. Instead the shutter speed determines the exposure for the parts of the scene were the flash has the least impact (i.e. the shadows and the background).

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 fact that the shutter speed does not inpact on the flash's contribution to the exposure is the basis for a very useful technique known as “balanced fill flash”. The main idea behind this is to set a combination of shutter speed and flash output power that “balances” the light from the flash with the ambient light, giving you a scene where both the shadows and the highlights, the foregound and the background, receives exactly the right exposure.

The TTL BL setting on the Speedlight is to optimize the TTL metering for this type of exposure. We go through using the TTL BL mode and how you juggle shutter speed and aperture to acheive this in more detail in case #1 below.

In situations were the ambient light is too low for balanced fill flash to be workable, you're best option is to use the flash as your main light. In that case, you want to reduce the impact of the ambient light on the scene, so you're best shutter speed is the camera's maximum x-sync speed (1/200th om most Nikon DSLRs). This is discussed in more detail in case #2 below, where we use the TTL mode.

In some situations where we want to use flash we need to use a shutter speed faster than the camera's top x-sync speed. In these situations, FP high-speed sync mode should be used. This is discussed in case #5 below.

Balanced Fill Flash or Flash Key?

In many situations, you might wonder whether to flash key (where the flash is your main or key light) or balanced fill flash. If you are filling in the shadows on a sunlit day, balanced fill flash is the obvious choice, but indoors, the choice may be harder.

Provided that your flash is powerful enough, you should always be able to use your Speedlight for as your main light. However, to use it for balanced fill flash in a dim setting may not always be an option.

analog exposure display

To check if you can use your Speedlight for balanced fill flash, try the following:

The exposure of the background will depend on your shutter speed. Select the slowest shutter speed you can handhold 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 get balanced 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 useable f-stops or ISO values without ever getting “balance”. In that case, give up trying for balanced 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 for balanced fill flash.

To use your Speedlight for flash key, you set the shutter speed to your camera's fastest x-sync speed (1/200 for the D80). This will minimize any impact of ambient light, so the flash will be your main light source. In that case, you should use the TTL mode.

For most subjects,balanced fill flash will give the best quality light, but if you need to eliminate mixed lighting (see case #2), or make your main subject stand out, using the Speedlight for flash key may be better.

Exposure compensation

If the histogram and/or review image on the camera's LCD screen reveal a problem you may, in some cases, get the exposure right by using exposure compensation.

You may do this on the camera and/or flash. The latter is known as flash output level compensation (FOLC). Care should be taken when you do this both places, as the effect is cumulative. In general, use FOLC in TTL BL to modify the exposure of the main subject, and on the camera to modify the exposure of the whole scene.

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 upping the 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 (may work best in TTL BL mode), 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.

3. 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: Creating a Balance Between Ambient Light and the Flash

TTL BLFlash 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 (balanced fill flash) 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 that are reflected from the grey shutter curtain in the camera onto the TTL flash sensor. When the shutter curtain lifts, the flash fires with a power output based on a computation of the ambient light metering and the the pre-flash measurement, so that the flash just “fills-in” the foreground.

Balanced fill flash is a highly sophisticated combined operation where the camera and flash work together to adjust the power of the flash to blend nicely with the ambient light. I never use green Auto or any of vari-program when aiming for balanced fill flash, because these programs give very little control to the photographer, but it can be used in all the camera's conventional exposure modes (P, A, S and M) and is compatible with both normal and rear curtain sync. However, for balanced fill flash, you must use 3D matrix metering (recommended) or centre-weighted metering. Spot metering will force the camera to TTL (balanced fill flash 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 for balanced fill flash. The distance information from the lens and 3D matrix metering is used to determine the subject/background flash ratio if the subject is not located in the centre of the frame. When using a lens that do not report distance information with balanced fill flash, the flash power calculation is based on a centre-weighted metering pattern and if your main subject is off-centre, use FV lock to lock flash exposure while having your main subject in the centre, then recompose.

By default, the maximum shutter speed in the P and A exposure modes are restricted to maxiumum 1/60th second 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 selecting the M or S exposure mode (in which case you pick the shutter speed), or by dialing in slow sync flash.

By setting the shutter speed long enough, you may be able to capture enough of the ambient light to get the exposure right for balanced fill flash. However, this may result in shutter times unsuitable for moving subjects, or may make it necessary to use a tripod.

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 more problematic conditions for balanced fill flash are outlined below:

In such situations, other techniques and settings, some of which are described below, may prove to be more effective.

Some people has fast enough reflexes to blink when they are exposed to the pre-flash used to compute exposure in TTL BL mode (and other modes that uses pre-flash). You can avoid this 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.

Case #2: Reducing the Effect of Mixed Lighting

TTLOne of the factors that digital photography has introduced is the issue of white balance. This is known to film photographers by the need to match the film type with the light (daylight film, tungsten film) or to add filters to eliminate the green cast of fluorescent light. Digital cameras are able to automatically or manually compensate for the colour imbalance caused by different light sources and accommodate many types of lighting. However, there are limitations to the range of compensation, and one major consideration is when different light sources are present. For example, daylight from a window or strong overhead 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 these conditions, it may be desirable to reduce the effect of the ambient light and dominate the scene with the light from the Speedlight.

In this situation, use the highest possible shutter speed that allows syncing with the flash at full power (on most Nikon DSLRs, this is 1/200th of a second) to minimize the impact of the ambient light.

In TTL mode, pre-flashes are used to measure the reflectance of the scene in centre-weight mode. When the shutter curtain lifts, the flash fires with a power output based on the pre-flash measurement. Because TTL is heavily biased towards the centre, if your main subject is off-centre, use FV lock to lock flash exposure while having your main subject in the centre, then recompose.

Other Uses of Plain TTL

In situations where the subject should stand out strongly from the background, you should also use TTL. 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. Also, when doing a portait shot outdoors at night or in a large, dark room, TTL will be more appropriate than TTL BL.

Case 3: Bounce Flash

AWhen the flash is mounted on the camera and the head is pointed directly at the subject, the resulting shadow may be visually objectionable, in particular if the subject-to-background distance is small, or if you shoot with the camera turned 90 degrees for portrait format without using a flash bracket to shift the flash above the camera.

Tilting the head to bounce light from the ceiling will result in softer illumination as the light is reflected from the large ceiling area instead of hitting the scene head on. This is known as “bounce flash”.

When using bounce flash, many report that they get more accurate exposure with the flash in Auto (A) or Auto Aperture (AA) mode than when using TTL BL or TTL.

Caveat: Note that the reflected light falling on the subject will be coloured by 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 take with white balance. Turning the camera by a few degrees may bring another surface into play which could influence the colour of the scene. The D80 has many white balance control options. You may also make use of a WhiBal or similar white balance card for setting accurate white balance.

Other Uses of non-TTL Auto

Because the Auto-mode does not use 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.

Some people have fast enough reflexes to blink when they are exposed to the pre-flash. By using Auto-mode, you can photograph such people with their eyes open.

Note: If you use the very similar Auto Aperture (AA) mode on the Nikon SB-800, it will emit a preflash. If avoiding pre-flash is your reason for selecting this mode, make sure the mode you select is Auto (A).

Case 4: Working in Controlled Conditions

MMost 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 paramteter – 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 rewiew 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

TTL BL FPThere 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 use a shutter speed faster than the camera's x-sync speed (1/200th om most Nikon DSLRs) to get the aperture you want. In this case a special setting called FP lets you set any shutter speed you want. Note that using FP reduces the maximum power of the flash.

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 traveling slit of a dual curtain shutter passes over the sensor at high-speed.

To use this mode on a Nikon D80, you need to enable Auto FP (custom setting #25). Then, set the camera to manual exposure mode and raise the shutter speed to greater that the x-sync speed. The flash will then add FP to most other modes except A.

High Speed Sync Without FP mode

Not all Nikon DSLRs support FP mode. However, the following DSLR models: D70, D70s, D50 and D40 (but not the D40x) will sync with most generic units at any shutter speed (i.e. up to 1/4000th 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 1/125th of a second.

For some reason, Nikon has put in a program limitation that will not let you set faster shutter speed than 1/500th of a second in these models if you mount a dedicated flash unit in the camera's hotshoe. You can get around this limitation by taping over the two metal contacts on the back of the Speedlight.

4. Pointers

The first installment of this series dealt with buying a flash for your Nikon system.

In the next installment if this series (forthcoming), I will discuss AWL and how to use CLS with several Speedlights.

Edwin Leong:
Flash, Flash technical (Basic tutorial on flash photography.)
Neil van Niekerk:
Flash Techniques (Tutorial on flash photography for weddings and portraits.)
Nikon:
The World of Nikon Speedlights (Nikon official site with tutorials and examples.)
Ralph Paonessa:
High Speed Sync for flash at any shutter speed (Introduction to FP flash use for Canon and Nikon)

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