iddi wrote:
>I found an old Vivitar 2500 (zooming head, bounce/tilt, 2 auto exp, full,
>and 1/8 power) flash at work in the rubbish pile, and immediately "saved"
>it with permission of boss.
>
Free is always a Fang, if the item works.
>It will be a nice supplement to the T32.. Now,
>what's the guide number? The users' guide makes no mention of it, and
>googled values range from 50 feet to 120 (which seems awfully high,
>certainly it doesn't appear stronger than the T32). Anyone have a definite
>number? I'm guessing 50ish, at the 50mm setting, ISO100. The 1/8 power
>feature, coupled with bounce, might work well for indoor candids... Will
>try it out w/ a roll of b/w. Thanks.
>
The guide number will, of course, vary with zoom setting. The easy way
to figure out the GN of any flash with a manual setting is to set iso to
100 in manual, read the distance opposite f2 and multiply by 2. So, for
example on a T32 in full power manual mode, the calculator panel reads
50 ft. at f2, so the GN is 100. On a Viv 550FD, the calculator reads 12
m and 40 ft. at f2, so the GN is 24 in meters and 80 in feet.
This derived from the definition of GN. GN was devised long before
electronic flashes, let alone auto flash, to allow easy calculation in
one's head of the correct f-stop for flash. Given the GN for your film
speed, you just divide the GN by the distance to get f-stop. So you can
just use the formula in reverse to find the GN. Since calculator panels
don't have f1.0, I use f2 and multiply the resulting distance by 2.
Moose
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