Determining Guide Numbers

[Christopher Walrath]

I have an old VIvitar 3800 auto thyristor flash unit. Been using it since 1996, back before I knew what a guide number was, let alone had a use for one. Well, here I am and no owners manual to point me in the right direction. Is there a way for me to determine a flash's guide number manually? And I tried Craig's camera, butkus, and all of the other sources I could come up with, even Vivitar (nothing non digital there, go figure). All help is always appreciated. Thank you.

 

[David H. Bebbington]

I have an old VIvitar 3800 auto thyristor flash unit. Been using it since 1996, back before I knew what a guide number was, let alone had a use for one. Well, here I am and no owners manual to point me in the right direction. Is there a way for me to determine a flash's guide number manually? And I tried Craig's camera, butkus, and all of the other sources I could come up with, even Vivitar (nothing non digital there, go figure). All help is always appreciated. Thank you.

The obvious answer is: Use a flash meter! Assuming you don't have, or have access to, one of these, very often model designations for flashguns have the GN for ISO 100 (meters) hidden within them. In your case, I would say the GN could very well be 38!

Regards,

David

 

[Bob F.]

Assuming it has a manual mode, you should be able to work it out from the manual settings slider (or whatever it has) using the formula: GN = f-stop * distance. So, if you set the manual settings for 100ASA film and 10 feet distance and it tells you to use f/8 then you have GN = 10*8 = 80 (feet) at 100ASA.

Bob.

 

[David A. Goldfarb]

Or if you don't have a flash meter, you can test with film, which is what I used to do before I had a flash meter. It's easiest to use slide film, unless you've got a well-calibrated system for neg film (i.e., you've done speed and development tests and can accurately interpret your GN tests).