>
> But a guide number won't do you much good for a fill flash.
>
Why not?
A guide number provides a known element in the equation for proper exposure
that includes the following:
1. ISO sensitivity of the film or digital sensor
2. Ambient light level (basis for everything)
3. Effective flash brightness
4. Subject-to-flash distance.
My working method would be something like this for outdoor fill on bright
sunny day:
1. Determine the ISO I'm going to use for this--in lower light conditions or
indoors, steps 1 and 2 are sometimes recursive. For bright sunny day, I'm
usually at ISO 100.
2. Measure ambient light level and place the exposure of the ambient high,
low or at the "proper" exposure. If I'm balancing the light in such a way
that the ambient is dead-on, then it will look something like a Sunny-16
exposure, or inotherwords some equivalent to F16 at 1/125. However, that
means that with a camera like an OM with 1/60 flash sync speed, that makes
it F22 at 1/60. So, I may grab the polarizer to knock off two stops and give
myself F11 at 1/60, but when using film I prefer to run my ambient either a
stop high or a stop low, so I'll do F8 at 1/60 with polarizer. With digital
cameras, we have a little more flexibility, so my base exposure of F16 at
1/125 can go up to 1/180 or so, and with a polarizer, I can get back down to
the F5.6-F7.1 region. (this is one area where the A1 shines because of the
almost unlimited flash-sync speeds). But, for this example, let's stick
with F16 at 1/125 and assume that it works with the camera.
3. A guide number is essentially the following equation: "GN = Distance *
F-number". We have two numbers already. The published or calculated or
metered GN of the flash, either electronic or bulb. .
4. We have the F-number from step #2, therefore, to determine flash distance
from the subject we use the following: Distance = GN / F-Number. (you will
need to make sure you are working in feet or meters). So, based on this
calculation you will know exactly how far away from the subject to place
your flash.
Or the simplest way is to waste one bulb with a flash meter and adjust
distance by estimation.
AG (I don't even play Dr. Flash on television) Schnozz
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