Flash Exposure Compensation
(FEC) is used to adjust the flash exposure in cases where the subject
is not middle-toned, such as someone wearing a white wedding dress,
or another person wearing a black tuxedo.
FEC is available as a feature
on some dedicated flash units, and also on most EOS camera bodies.
If you shoot film with an EOS KISS/REBEL body, you can use Manual
exposure and ISO indexing to fool the camera into adjusting the
flash exposure appropriately.
If your camera and flash
lack FEC and also cannot make use of ISO indexing (e.g., EOS 300D
/ Digital REBEL), it is still possible to fake FEC by misusing
the flash exposure lock (FEL) feature to achieve a usable degree
of flash exposure compensation.
The trick is to apply FEL
to force a flash meter reading and fix the flash power. By
changing the flash-to-subject distance or the reflectance of the
FEL target, a different flash exposure will result. Voilà, flash
exposure compensation!
Light intensity decreases
by the inverse square law of physics. Once you have locked the
flash intensity using FEL, you can make fine adjustments simply
by changing the flash-to-subject distance. Halve the distance,
and get 2 stops brighter light. Double the distance, and get 2
stops darker. For one stop or finer intervals, just remember the
familiar number series from your aperture scale.
One stop
1
1.4
2
2.8
4
5.6
8
11
16
22
32
45
64
Half stops
1
1.2
1.4
1.8
2
2.5
2.8
3.5
4
4.5
5.6
6.7
8
9.5
11
13
16
19
22
27
32
38
45
54
64
Third stops
1
1.1
1.2
1.4
1.6
1.8
2
2.2
2.5
2.8
3.2
3.5
4
4.5
5
5.6
6.3
7.1
8
9
10
11
13
14
16
18
20
22
25
29
32
36
40
45
51
57
64
If you are using Fancy Footwork
and standing 4 feet away from your subject, FEL at 5.6 feet (one
step
back) will give you +1EV FEC if you shoot at 4 feet. Conversely,
FEL at 2.8 feet (one step forward) will result in -1EV FEC for
a shot taken at 4 feet.
Need this in metric? For
shooting at 1.4 m: FEL at 1 m for -1EV FEC; or at 2 m
for +1EV FEC. Or choose any ratio that is 0.717x, 1x, and 1.414x.
As
usual, your camera, subject, or shoe size might be different, so
proof check your results before committing to an important shoot.
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