Flashbulbs

[P3D David W. Kesner <drdave@xxxxxxxxxx>]

Just some added info on flashbulbs, their guide numbers , and use.

First, I have a very detailed chart if anyone is interested in
knowing a guide number for a specific bulb. Email me and I will
answer your question.

What hasn't been mentioned is that reflector type and shutter speed
effect guide numbers. For instance take a Sylvania  Press 25B bulb.
Assuming the film used is ASA 80-125 and you are using a 4-5 inch
polished reflector and 1/30 shutter speed then the guide number is
200. A satin reflector needs 1/2 f-stop more (don't start the 1/2
f-stop thread again) and folding fan reflector 1 full f-stop. If the
shutter speed is bumped up to 1/125 then the guide number is 160 and
100 if the speed is 1/250.

When using the formula guide number divided by distance equals f-stop
don't forget that the distance is from the bulb to the subject not
the camera to the subject. If you do a lot of flash shots with the
flash away from the camera, then you must take that into
consideration. Another consideration is the reflectivity of the
subject you are flashing. In my case a dark lava tube cave will lower
the guide number as oppose to the same distance in a bright limestone
cave. This can be troublesome if you have several different surfaces
in the same frame, but as someone else has said being able to figure
that out makes "the difference between a photograph and a snapshot".
If you want to use flashbulbs as a fill when shooting outdoors you
can use the "reverse method". Set your camera to whatever is called
for (ie f8 at 1/100) then divide the guide number by the f-stop and
you have the distance in feet of where you should fire your flash.

Thanks for the time,

David W. Kesner
Boise, Idaho, USA
drdave@xxxxxxxxxx
(No I'm not a doctor - just a nickname)


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