"george" <geanders@xxxxxxxxx> wrote>
> "Two T32's=GN64"
>
> This is not true. Guide Number is a measure of distance the flash is capable
> of lighting up, assuming some fixed ASA/aperture/shutter speed. 2 T32s will
> be able to throw more light on a subject at a distance of 32 meters, but
> none of that light will reach a subject at 64 meters.
>
Be careful how you think about GNs! The light does not magically stop
at 32 meters.
Some of the light from a t32 will reach 64 meters. Heck, some of the
light from a t20 will reach 64 meters. However, because it is spreading
out its intensity falls with the square of the distance and thus
only 1/4 the power (per surface area) reaches to 64 meters. Assuming
the GN is accurate (which depends on the actual environment of the
photo, but for these purposes will be a reasonable assumption), at 32
meters the t32 will give sufficient exposure for 100 speed film at f/1.
At 64 meters, there is only 1/4 the power, but you could make up for
that by somehow opening up two stops -- either by opening your aperture
to 4f (that is, f/0.25) or (more likely) by switching to 400 speed film.
If you add a second t32, you will get twice the flash power at 64
meters, so you'd only need to pick up 1 stop through aperture choice
or film speed. Four t32s would get you 64 meters at iso100 and f/1.
Note that since the GN is determined for an f/1 aperture setting,
it'll be difficult to actually use a t32 as primary illumination at
32 meters. I don't think there are any 300mm/1.0 lenses in the
Zuiko lineup.
:-)
joey
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