Converting Guide Numbers from 4" to 6" reflector to 7" reflector

[Sirius Glass]

I have a 2, 3,5 battery Heiland Research Flashgun with a 7" reflector and a bayonet adapter 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?

See my calculations below.

Steve

[E. von Hoegh]

How do you get 19.63 sq.in. for a "4" to 6"" reflector?

Multiply your guide number by the ratio of areas of the reflectors.

[Sirius Glass]

How do you get 19.63 sq.in. for a "4" to 6"" reflector?

Multiply your guide number by the ratio of areas of the reflectors.

Yes, that is what I meant.

[Bob Salomon]

But the GN for any diameter reflector will change with its shape and finish. Are they both shiny surface flashes with the same shape or is one more pancake shape? Is one matte or pebble finish?

[Sirius Glass]

Both shiny and both assumed to be the same shape.

[Bob Salomon]

The Kodak Professional Photoguide shows that with a "Polished Deep Bowl Reflector" and a 25 bulb you get one stop more light then a 25B or 5 B in the same reflector and 2 stops more light then a 5B or 25B in a "polished Shallow Bowl or Polished Fan Type Reflector"

It also states that the guide numbers that they indicate were based on a 4 to 8" diameter shallow bowl polished reflector and a 4 to 5" deep bowl polished reflector.
So if you have a shallow bowl reflector it won't make any difference in the exposure.

[johnielvis]

SIZE in this case matters LESS than the focus of the beam---you can have a teeny tiny reflector with the bulb out of the focus so that it's beam is spread all over for a smaller guid number than a larger reflector focused parallel....this is something best measured by a regular bulb in the reflector and a light meter at the distance you're interested in...use those measured ratios---that's how I'd do it...but that's just me.

[Sirius Glass]

I want the comparison of 5" Graflite reflector and 7" Graflite reflector. The same reflecting surface and assuming the same shape. So assuming the same shape is the mathematical calculation that I proposed correct or is there a better model?

Steve

[E. von Hoegh]

You could screw a 120v lightbulb in and check with a meter, as has been suggested.

Or you can calculate the areas of the reflectors and find that the 7" reflector has almost exactly twice the area of the 5" reflector, thus making it one stop brighter.

There isn't much else you can do except to pop a bulb and measure it with a flashmeter, or expose a sheet of film.

[Bob Salomon]

"find that the 7" reflector has almost exactly twice the area of the 5" reflector, thus making it one stop brighter."

Only if they have the same beam spread. The narrower the beam spread the greater th output within the covered area.