[photo-3d] Re: Flash Photography help

["David W. Kesner" <drdave@xxxxxxxxxx>]

From: "David W. Kesner" <drdave@xxxxxxxxxx>

Since no one has really answered the question that was asked 
I'll take a stab.

If you are going to use your flash manually the first thing 
you must do is find the "true" guide number (GN). Do not 
believe what the manufacturer states, it can vary quite a bit 
and is somewhat dependent on the camera/lens/film you are 
using.

To find the GN go outside on a really dark night with the 
camera/lens/flash you will be using. Have a model dressed in a 
multi-colored outfit stand exactly ten feet away from the 
camera/flash. Do not have them stand against anything but lots 
of open space and do not have anything else close by. Take a 
series of images (using slide film only) at various fstops. 
Have the film processed and view the images to see which one 
is the best exposure. Look for bright colors in the outfit 
without being washed out. Determine which fstop that was taken 
at. Multiply this fstop by 10 and you have your GN. For 
example if f5.6 was best the GN would be 56. If f11 was a bit 
washed out and f16 was a bit underexposed your GN would be 
somewhere between 110 and 160 (where would depend on how over 
or under exposed).

No that you have the "true" GN you can determine the fstop for 
any given situation by a simple formula of fstop=gn/distance. 
So your GN is 56 and your subject is 12 feet away. fstop=56/12 
or 4.6 which would be f4. Your GN is 100 and your subject is 
12 feet away. fstop=100/12 or 8.3 which would be f8.

Most cameras have a scale on them that allow you to see the 
fstop at any given distance. However that is based on their GN 
and not necessarily your "true" GN. To set it to yours simply 
place the fstop you came up with at ten. If thier GN was 40 
the f4 would be above the 10 foot mark, if you GN is 56 slide 
the 5.6 over the 10 foot mark. Now all the rest of the numbers 
will be right.

GNs are based on flash to subject distance and NOT camera to 
subject. If you are using a flash that is not mounted on the 
camera you need to remember that. You can use this to your 
advantage. Say you are 15 feet away from your subject - with a 
GN of 56 you would need to use an fstop of 4, but want to use 
a smaller fstop for greater DOF. By moving the flash to 10 
feet away you could use f5.6 or move the flash five feet away 
and use f11.

To calculate this setup use the formula distance=GN/fstop. So 
if you want to use an fstop of 8 and your GN is 56 then 
distance=56/8 or 7 so place your flash seven feet away.

It gets a lot more complicated with multiple flashes. We can 
talk about that latter.

Hope this helps. If you have more question I can try to answer 
them when I return from a trip to Denver on Sunday.

That's all for now,

David W. Kesner
Boise, Idaho, USA
drdave@xxxxxxxxxx

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