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

Discussion in 'Lighting' started by Sirius Glass, Sep 3, 2011.

1. ### Sirius GlassSubscriber

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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

2. ### MattKingSubscriber

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Steve:

Do you have a flash meter? If so, using it and wasting a couple of bulbs would be a more reliable way to calculate a new Guide Number than any mathematical conversion.

Looking at one of my old Kodak Master Photoguides, it seems that Kodak uses reflector shape rather than size to aid in predicting flash to subject effects on exposure.

3. ### BrianShawMember

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Steve,

I assume you are refering to Graflite reflectors. I've had the same thought but always end up just using 5/25 bulbs with the small reflectors and 11/22 with the large reflector. Flash calculations invovle many variables and can get complex so experimentation/experience seems the best answer... of which I have NONE when it comes to mixing/matching bulbs and reflectors.

I also am wedded to my collection of old Kodak Professional and Master Photoguides because of their handy flash calculators and rely on them as I think through flash bulb applications.

Here are some numbers to think about from the 1977 Professional Photoguide:

25B bulb @ 100 ASA for deep reflector (4 - 6 inch) - GN = 175
25B bulb @ 100 ASA for shallow reflector (4 - 8 inch) - GN = 120

In my kind of arithmatic I'd open 1 stop if using the 7 inch reflector vs the 5 inch with a 25B (for example) bulb.

Take this with a grain o'salt since it's purely an academic comment with no exeprience or success to justify it!

4. ### BrianShawMember

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Matt, you are right. That makes for a frustrating situation! The Kodak Professional photoguide was a little more descriptive and linked a size range with the shape descriptor.

5. ### Ian CMember

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If you want to be accurate, you can use a flash meter at a fixed distance from the bulb using each of the reflectors with the same bulb size in both measurements.

Once you know the difference in stops between the two reflectors youll have all the information you need for any bulb size.

For example, suppose the smaller reflector gave you f/8 + 0.6 stops and the larger reflector gave f/8 + 0.1 stops.

Then the difference in stops = 0.5 = Δf. The reflector-to-flash meter distance is d.

The difference in guide numbers is

ΔG = Δf*d

Example:

Suppose that d = 12 feet. Then

ΔG = 0.5*12 feet = 6 (in feet)

6. ### Sirius GlassSubscriber

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No, I do not have a flash meter

Yes, a 2, 3,5 battery Heiland Research Flashgun with a 7" reflector and a bayonet adapter.

My calculations seem to bear this out. See attached.

I stopped using salt years ago when I cook because the salt need is more than made up with commercial prepared and restaurant food.

When do you #11 and #22 bulbs?

Steve

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7. ### Ian CMember

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If you don’t have a flashmeter, you could obtain the same results by temporarily using a continuous-light bulb of a similar size in the same position. Then you’d perform the measurement with a standard light meter.

This should determine the reflectance differences between the two different sized reflectors.

8. ### BrianShawMember

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On the rare occasion that I'm taking a "wide angle" shot and want to use the 7 inch reflector for greater coverage.