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Understanding Flash
Photography
(C) 2005 Angelo Lorenzo
Many amateur photographers are afraid of flash photography. Its
that little flash of lightthat fills a dark scene but doesnt make
the photo look too good. Here we will explorehow to use a flash
properly.
This document may not be published elsewhere, in part or in
whole, or modified.
What is a flash?A flash unit, or an on camera flash both work in
the same fundamental way. Voltage froma battery is stored in a
capacitor, and then as a photo is taken, the voltage is released
to
the flash tube filled with a gas and produces a flash of light.
This burst of light may onlylast from 1/1,000th of a second to
1/50,000th of a second depending on your flashesmodel.
There are three types of off camera flashes: manual, automatic,
and dedicated. With
manual flashes the usually fire their flash with 100% intensity
and you have to set yourcamera accordingly. Automatic flashes will
cut off the amount of power in the flash onceif feels the subject
reaches a proper exposure, but you still have to set your
camera.
Dedicated flashes will usually communicate with the camera and
set both the flash andthe camera settings. As each flash becomes
more automatic, the price will usually rise,
and for the sake of this tutorial we will be discussing certain
aspects like all you have is amanual flash.
All About Guide NumbersNow every flash is rated with a guide
number, whether on-camera or a separate flashunit. The guide number
is the distance the flash can cover. A guide number may be inmeters
or feet or both, so just stick with one unit for all your
measurements and you'll be
fine. Guide numbers are rated at ISO100 which you'll see makes a
difference later. I warnyou this section has some math so put your
thinking helmets on.
Now lets say we have a flash unit and its guide number is 120ft
(approx 40m.) and wehave our subject 20 feet (6.6m) away. All you
have to do for proper exposure is divide
the guide number by the subjects distance and the result is the
proper f/stop to use. Nowthe number you get may not be an f/stop
you can select so its best to round your number
to the closest f/stop available to you. Example:
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120/20 = 6 or40/6.6 = 6
Now the closest f/stop to the number 6 would be f/5.6 so to
properly expose our subjectwe'd use f/5.6. As you increase your
ISO, you increase your guide number. Doubling
your ISO increases your guide number by 1.4 and quadrupling it
increases your guide
number by 2. So at ISO200 a guide number of 120ft may be 170 or
it may be 240ft atISO400. Also at ISO50 it may be 86ft
(120/1.4)
Shutter Speed and FlashNotice how I haven't mentioned much about
shutter speed yet? Shutter speed does notcontrol a flash's
exposure, instead it controls what we call ambient light. Say
we're
shooting at night with a train station behind us. If we shot at
1/60th of a second with aflash, the train station may appear dark,
while shooting at 1/15th of a second may make
the train station appear properly lit.
You may also have noticed that with a flash you cannot go above
a certain shutter speed
(usually 1/60th or 1/125th of a second). This is called flash
sync. SLR cameras have acurtain shutter which is comprised of 2
moving curtains. As one opens and exposes the
film for the proper amount of time, the second releases to cover
the film again. Thissystem allows for very fast shutter speeds but
if the flash doesn't fire at the moment bothcurtains are wide open
(the sync) then parts of the image will be black because the
shutter
was blocking the frame. Medium Format cameras and some 35mm
cameras have leafshutters which open in a circular configuration.
Leaf shutter systems are usually more
expensive because the shutter is built into each lens instead of
the camera body. Theadvantage to leaf shutters is flash syncs at
all speeds but the disadvantage is a sloweroverall shutter speed
(perhaps 1/500th compared to 1/8000 for curtain shutters).
Digital
point and shoots can usually sync flash at all speeds because
they don't use shutters, theyjust sample from the sensor for a
certain amount of time, because it's always on.
Fill Flash and Bounce FlashNow these two situations are probably
the most complicated you'll face in the world offlash
photography.
Fill FlashNow say we're outside and we get a meter reading of
1/125th and f/8 for a properexposure, but our subjects are a bit in
the shade so we decide to use fill flash. Now weknow our guide
number is 120ft and at f/8 so the best distance for proper exposure
would
to have our subjects be 15ft (120/8) away from the camera... but
we wanna get closer. Ifwe increase our f/stop to f/16 we can be
7.5ft (120/16) away from our subjects, getting a
great portrait. Because we closed 2 stops we have to compensate
by slowing our shutterspeed 2 stops to 1/30th and we'll have a
proper exposure for the scene with fill flash.
Bounce FlashNow lets say you have a flash unit where you can
angle the head to bounce the flash off a
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wall for diffusion. We now have to measure the distance from the
flash to the wall, thewall to the subject and the subject to the
camera. We get these 3 values and divide them
by 2 to get an average distance we can plug into the guide
number formula. Lets say thedistance from the camera to the wall is
3 ft, the distance from the wall to the subject is 5ft
and the distance from the subject to the camera is 8ft.
3+5+8=16/2=8. Now we just take 8
and plug it into our guide number equation to get an f/stop of
f/16 (we're still assumingfor example purposes that our guide
number is 120ft). Now because we're diffusing the
light some we'll end up losing a bit so its wise to open up a
stop to compensate whichmeans we'd be using f/11 for this shot.
