I have a 7" flash reflector and the Guide Numbers for the 5, 5B, 25, and 25B bulbs with a 4" to 6" reflector. How do I convert the Guide Numbers from a 4" to 6" reflector to a 7" reflector?

Should I take the area for a 4" to 6" reflector of 19.63 square inches and divide it by the area of a 7" reflector 38.48 and multiply by the Guide Number?

7" Guide Number = 5" Guide Number*(19.63/38.48 )

Or should I use the 4" to 6" Guide Numbers?

Steve

hey you can take a look at http://graflex.org/flash/navy/

1banjo wrote:

hey you can take a look at http://graflex.org/flash/navy/

I read that before I posted. It does not give an indication of the reflector size for each bulb or even all the bulbs.

The manufacturers' published material for #5, #5B, #25, and #25B all are rated for 4" to 6" reflectors. Should I blow off the difference with the 7" reflector or compensate for it? If I should compensate, how?

Steve

The difference should be roughly one stop. That is you will get less light and a wider angle with the larger reflector.

mopar_guy wrote:

The difference should be roughly one stop. That is you will get less light and a wider angle with the larger reflector.

That agrees with a posting on Large Format Photography Forum and the calculations in my first post. No one on any of the forums disagree. I think I see a pattern here ...