Photography Techniques: Photo Light & Flash -> Flash Photography
- Controlling the Background Exposure
Flash Photography - Controlling the Background Exposure
photography is one of the most misunderstood areas of the art. To many people,
the flash is a mystical device that, while easy to use (it attaches it to your
camera’s hot shoe), is difficult to use correctly in many situations. One of
the common questions in my classes is “Why did the background turn black?”
If you’re like
most photographers, many of your flash photos come out fine. However, you
create a significant percentage of flash photos in which the background appears
dark or completely black. This article will not deal with properly illuminating
the subject using either full or partial (fill) flash. Instead, it will provide
information to assist you in properly illuminating an interesting background
while using your flash on the main subject. Although one technique we’ll cover
requires that you use your camera in manual mode, the information presented here
is designed for photographers with auto focus cameras.
There are two
primary causes for a dark background in flash photography: flash-background
distance and the exposure mode. Discussing flash-background distance requires a
brief summary of the basics of flash photography.
(ambient) light photography, the two most important controls determining
exposure are f-stops and shutter duration. (We’ll use the term “shutter
duration” because it’s more accurate than “shutter speed.”) Technically, the
shutter curtains open and close at the same speed no matter what the length of
the exposure is. In flash photography, shutter duration is not relevant.
shutter duration is important in one aspect. For every camera, there is a
shutter duration that’s designated as the maximum “flash synchronization
speed.” You can achieve properly exposed flash images by shooting at the
maximum flash synchronization speed or any slower duration.
If you shoot at a speed faster than the maximum flash synchronization
speed, the shutter will close before the exposure is complete. This will result
in a black band along the top of your image.
cameras, the maximum flash synchronization speed is a different color on the
shutter speed dial from the other numbers. The maximum synchronization speed
for auto focus cameras is not a significant issue. Auto focus cameras will not
let you shoot faster than the maximum flash synchronization speed. There are
many auto focus camera/flash combinations that allow you to shoot at any shutter
duration available on your camera. This feature is most often called High Speed
photography, the two most important controls over exposure are f-stop and
flash-subject distance. These controls combine to create the flash unit’s Guide
Number. The guide number = f-stop x flash-subject distance. For instance,
GN = 10' x f/4 = 40.
The guide number is a relative indicator of the power of the flash. A flash
having a guide number of 150 is more powerful than a flash with a guide number
of 100. However, be careful when comparing guide numbers. Some are listed in
feet; others are listed in meters. For proper comparison results, be sure each
flash is listed in the same measurement and uses the same focal length lens and
ISO. (Most guide numbers are computed at ISO 100 with a 100mm lens.)
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number can be used to ESTIMATE either the f-stop to use or the distance the
flash will cover. Consider a pop-up flash, which normally has a guide number in
the low forties, as an example. For ease of calculation, let’s use forty (in
feet) as the guide number. To determine how far the flash will extend, divide
the guide number by the f-stop. If we use f/4 with a flash having a guide
number of forty, our flash will extend only ten feet. It’s only five feet if we
use f/8 to get more depth of field. Background objects in the extra depth of
field we desire will be underexposed due to the short distance the flash covers. Obviously, your pop-up flash is not very powerful, so don’t use it when you’re
seated in the third deck of a baseball stadium trying to take a photo of the
guide number by the flash-subject distance will give us the f-stop to use. If
we know the subject is ten feet away, then forty divided by ten equals f/4. If
you want to know if the flash can extend a certain distance past the subject,
replace the flash-subject distance with the total distance in the formula.
were developed experimentally indoors in a highly reflective room. Unless
you’re shooting under those conditions,
consider the guide numbers to be estimates. This is especially true when you’re
using the guide number to calculate either the f-stop or flash distance. In the
example above, we calculated that the flash would extend ten feet when we were
shooting at f/4 with a guide number of forty. In typical field shooting, the
flash would probably not go that far. Use a slightly shorter distance, such as
eight feet, to ensure a good exposure.
ISO, the other
control over ambient light exposure, does have an impact on the guide number.
The guide number increases as the ISO increases, because it is more receptive to
light. If the guide number were computed with ISO 100, a lower ISO would have a
lower guide number; an ISO greater than 100 would have a larger guide number.
systems are coordinated so that the point of focus is also the point of exposure
for the flash system. This is logical. The point of focus is the main subject
in most photos, and you want the main subject properly exposed by the flash.
There’s only one exception to the guideline “your subject will always be
properly exposed if it’s within the range of the flash.” Each flash unit has a
minimum distance that the subject has to be away from the flash in order to
enjoy proper exposure. Macro flash units have a very small minimum distance.
Normal flash units may have a three-foot minimum distance. Check your flash
manual for the minimum distance, or your subject might be greatly overexposed.
In all other
situations, the flash should properly illuminate your subject, if it’s within
the distance the flash will extend. However, light falls off rapidly at the
square of the distance. If your background is any significant distance from
your subject, the wrong exposure mode may result in the background not receiving
enough illumination from the flash.
where you want to illuminate the background as well as the main subject,
estimate the distance from the camera to the background element(s) that you want
illuminated. Use the Guide Number formula for the f-stop you desire to see if
the illumination will extend to the background elements. If it won’t illuminate
the background, you may need to open up the aperture and try again.
The second cause for dark background is the exposure mode. Which auto exposure
mode works best with flash photography? It depends. Let’s examine the
differences between shooting in Program Mode (P) versus Aperture Priority Mode
(A or Av). Note that some cameras have two different program modes. One is
usually a fully automatic mode; the other one is an automatic mode that allows
the photographer to make changes to the settings. Many Canon cameras, for
example, use the “Green Square” as the fully automatic mode. The “P” setting is
also automatic, but it allows the photographer to make changes.
In most cameras, all of the program modes become fully automatic and don’t allow
the photographer to make changes in the way they could in normal ambient light
photography. The camera will provide an f-stop and shutter duration that you
have to use—no exceptions.
Due to the bias toward shutter duration in all program modes, the camera will
set a flash synchronization speed anywhere from 1/60th to the maximum
flash synchronization speed. Remember, you can’t change the shutter duration to
a slower one. Exactly which duration is selected will depend upon the level of
In situations where the ambient light level isn’t too dark, Program Mode does an
excellent job of controlling flash exposure. However, when the light level
drops (you can tell because the shutter duration will be at 1/60th),
the minimum shutter duration of 1/60th may not be long enough to
capture the background ambient light. The result is that your subject will be
properly exposed, while your background will be dark or even black.
Aperture Priority Mode automatically goes into a “slow-synchronization” mode
when used in flash photography. This mode automatically allows the camera to
select shutter duration (as long as it is 30”, if necessary) slow enough to
fully capture the ambient background light, while the flash properly illuminates
the subject. In low light situations, where you want to capture the background
ambient light, Aperture Priority Mode is the best choice. Be aware that it can
easily give you a shutter duration of 1”, 4”, or longer. A tripod, or some
other way of stabilizing your camera for a long duration, is absolutely
Look at the
three images below to see the difference between Program and Aperture Priority
exposure modes. Each image was taken from the same location with the focus on
the illuminated wall in each photo.
The first image
was taken without a flash in aperture priority mode. You can tell that a flash
wasn’t used, because the color of the sign is influenced by the color
temperature of the fluorescent lights that illuminate it. As you can see, the
interior of the building in the background is hardly illuminated--even though
the camera provided long shutter duration at this low light level.
A flash was
used in the second photo. Note the difference in color between photos 1 and 2.
The flash approximates the color temperature of daylight, so the wall looks
similar to the way in which our eyes would see it. The illumination from the
flash extended beyond the wall. The reflections of several metal window supports
are visible due to a little more available background illumination.
The third photo
uses Aperture Priority mode and slow-synchronization. Notice the illumination
of the wall. It received both flash and more ambient light illumination.
Therefore, the wall in the image looks exactly as it does to the eye. The
longer shutter speed (4” versus 1/60th) enabled the building’s
interior to be properly illuminated.
You can use
Aperture Priority mode with flash for many situations. The mode works well when
you’re taking a photo of a person with a lighted building or other interesting
illuminated objects in the background. Landscape photographers can also use
Aperture Priority mode with flash. For example, allow the flash to illuminate
an interesting foreground object while a longer exposure captures the colors of
may want to intentionally lighten or darken the background. Perhaps the
background is cluttered or there’s an element that you can’t get completely out
of focus. In these situations, darkening the background allows you to place
more emphasis on your subject.
The easiest way
for auto focus cameras to do that is in Manual mode.
In manual mode, the flash will properly illuminate the subject. Any exposure
compensation that you perform will only affect the background.
If, for example, you use -1 stop of
exposure compensation, the background will be one stop darker, and your subject
will remain properly exposed.
To reduce exposure by one stop in Manual mode, establish the correct exposure
and either use a faster shutter duration or close down the aperture by one
stop. For example, if the correct exposure was 1/60th at f/5.6,
change the settings to either 1/125th at f/5.6 or 1/60th at
Since the flash will automatically properly illuminate the subject (if it’s
within the range of the flash), you can adjust the background illumination by as
many stops as you desire.
When using a flash in one of the auto focus modes, the flash performs
differently. For most cameras in an auto focus mode (aperture priority, for
example), any exposure compensation affects the subject as well as the
background. There is a work-around for this problem if your camera operates in
this manner. Suppose you want to deduct two stops of light from the background
but have the subject properly exposed. Start by using your exposure
compensation to reduce the exposure by two stops. This is easily accomplished
if you have an exposure compensation feature on your camera. At this point,
both the subject and background have two stops less exposure.
To compensate for the loss of two stops of light in the image, set either your
flash unit or camera’s FLASH exposure compensation to +2 stops. Increasing the
flash exposure by the same amount that you decreased the overall exposure will
properly illuminate your subject while making the background darker. For
example, decreasing the ambient light by two stops (through the exposure
compensation feature) results in the subject being illuminated at -2 and the
background at -2. Increasing the flash exposure by +2 results in the subject
being illuminated at 0 (-2 and +2 cancel out to give the normal exposure) and
the background remains at -2.
In summary, the distance from the camera to the background and the exposure mode
you use are the determining factors for background exposure. If you use an
automatic exposure mode, obtaining a shutter duration of 1/60th
should make you aware that the background may be dark. If the background is
important, switch to Aperture Priority mode, stabilize your camera for a long
exposure, and take the shot. There are legitimate reasons for darkening the
background, but it should be an intentional decision, not one achieved by
Jim Altengarten is the owner of exposure36 Photography. He specializes in photographic education through workshops, classes, private lessons, and classes on CDs. Information about these products is available at the exposure36 website (www.exposure36.com). He specializes in Canon EOS cameras (digital and film), and teaches several classes and workshops each year that enable EOS users to understand all of the functions of their cameras.
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