# Guide number for M3 flash bulbs?

Discussion in 'Lighting' started by kq6up, Jan 6, 2011.

1. ### kq6upMember

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I have just acquired a tilt-a-mite with some M3 bulbs. The guide number table on the back only gives old films that I am having trouble pining down the ASA values for these films that make sense with the data. The two films on the table that I was able to find ASA data for are:

Kodacolor-X ISO-80 GN=150 @(1/100s)

and

Verichrome Pan ISO-125 GN=240 @(1/100s)

The difference is 2/3 of a stop, so If I multiply 2/3*Sqrt(1/2)*GN it does not fit the data on the table. Sqrt(1/2)=.71 which is what you need to multiply to a guide number to account for 1 stop of film speed slower than the guide number. So I am confused. I think it is stupid they didn't put the film speed on the table.

Thanks,
Chris

2. ### Christopher WalrathSubscriber

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It is slightly off, only slightly. At f/8, the 125 would have a flash to subject distance of 30 feet. So that distance would be f/5 for the 80. So, off by about 1/3 stop in the ratings as a typical fstop number would be the standard 5.6. I would split the difference and call it GN=190 at ISO100.

Just what film are you planning to use? What format and lens? This can all factor in.

A guide number is generally GN=a(d) where a-aperture and d-flash-to-subject distance, assuming ISO100 and a normalish focal length lens. With modern guide numbers anyway.

3. ### MattKingSubscriber

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Are the M3 bulbs blue? The colour sensitivity of the film might factor in to the GN equation.

What are the other films on the table?

4. ### kq6upMember

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Here is a test shot ISO-125 @ f/11 1/30s The bulbs are not blue, and this is a RAW file untouched from my Canon 5D. I used a radio slave to protect the camera. I think the exposure was just about perfect:

http://www.flickr.com/photos/81639412@N00/5331665523/

I replaced the electrolytic cap with a new 200uF cap, and a fresh M504. I love it.

Chris

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I think the tilt-a-mite also allows you to NOT use the reflector, but position the bulb vertically and use it as open flash. In the 1960's I had a Braun wet-cell flash that had a removable reflector so I could use open flash, which bounces light all over and creates a more even light. Of course, because the light is not directed "forward" but spreads out spherically, your guide number is much lower.

6. ### Mike WildeMember

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The other thing to note is the longer burn time, so you can get more 'powerful' guide numbers by shooting at 1/30 or slower. I have about oh, 400-600 bulns in the M2 and 3, clear and B varieties.

PM me if no better info comes up. I have an old late 60's Kodak Photo Data Guide, or some other name for the guide book, with all sorts information useful to using flash bulbs in it like tables and dial calculators that I can scan and post.

7. ### Francis in VTMember

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IIRC the Guide Numbers for the M-3 bulb were for a 3" polished reflector.
When a folding Satin reflector like the Tiltamite was used the GN should be
multiplied by .7 . The other factor with Flashbulbs is the GN changes with the shutter speed as Mike wrote above.

Verichrome Pan ISO-125 GN=240 @(1/100s)
This GN would then be 168. YMMV

8. ### kq6upMember

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That is about what I used for the photo I sent the flickr link too. I should probably make my own table and publish it on the net, so others can have a better table. I hear the M3 bulbs have a fairly consistent output.

I have some AG1 bulbs too that did not come with a box. Any clue as to a guess on those little guys for ISO-100?

Thanks,
Chris

9. ### Mike WildeMember

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Get any old photography book circa 1970 and there are almost always flash guide number tables. The best is an old Kodak Master Photo Guide, with great little dial calculators for exposure with and without flash.