Flash - how to factor fl of lens into equation

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I have always used this formula for 35mm electronic flashes

GN/fstop x sqrt. (ASA/100) = distance

However, this formula does not take into consideration the fl lens being used... or the fl setting on the flash either. The formula assumes you enter the proper GN for the fl lens you plan to shoot with (and also the equiv. fl setting on the flash). Therfore when a Guide Number is quoted, it assumes a specific lens fl, some flash makers use 50mm and others like Cannon are more aggressive and use their high end zoom fl's, like 105mm. Some manuals produce tables showing the different GN's at each zoom level. However if I want to add a fresnel in front of the flash to decrease its angle of coverage to that equal to a 300mm lens, then how do I figure out the new GN for that flash at 300mm vs. say 105mm? I could not find a relationship between GN and fl in the manuals... for example, at 35mm fl the GN = 35, but at 105mm the GN = 54? I have never seen a formula take the lens fl (or angle of coverage) into account? Can anyone introduce this variable into the formula?

I realize for LF I would simply convert the 35mm to LF equivalents ... but to keep this question simple, lets just treat it as a 35mm issue. Any input would be helpful.... Thank you in advance...

-- Bill Glickman (bglick@pclv.com), April 08, 2000


Please go and talk to Wayne.

A guide numebr is simply a starting point. If you want a more accurate method of determining the exposure with flash then you need the BCPS or the ECPS and the angle that the number is given for. The BCPS and ECPS are based on flash coverage over a specific angle. They convert into a GN or a F stop.

-- Bob Salomon (bobsalomon@mindspring.com), April 08, 2000.

Or better yet, just make your own tests.

Each reflector, fresnel, diffuser, or other attachment to the flash, or change in power output (focal length setting on the flash unit in your case) changes the guide number. For a few years, before I could afford a decent flashmeter, I determined exposure for 2- and 3-strobe portrait and copy setups with nothing but a tape measure, paper and pencil, and guide numbers I'd determined objectively on film by bracketing exposures of a grey card and a scene with a variety of tones. It's somewhat labor intensive first time around, and it means planning all studio shots in advance, but that's not such a bad thing anyway, and I don't think I've ever made a wrong exposure in this way (which is not to say that I've never made an exposure error, just not by this method).

-- David Goldfarb (dgoldfarb@barnard.edu), April 08, 2000.

Rereading your initial post...

The actual focal length of the lens does not really matter, as long as the f-stops are accurately marked. The "focal length" setting on your flash unit is just a suggestion and is really a representation of the power setting of the flash. Some flash units mark this as a fraction of the maximum power of the unit, others in watt-seconds.

Once you determine the exposure for a particular flash-to-subject distance, you can change lenses all you want, and the exposure should be the same.

-- David Goldfarb (dgoldfarb@barnard.edu), April 08, 2000.

Thank you for the responses... I kind of left out one part of the situation... I was trying to calc. the new GN because I did not have the fresnels to accomplish a 300mm lens (or respective angle of coverage)

I agree with Bob about the BCPS, but I can not find that listed in the manuala? And assuming I find it, I still do not have the formula.

To clarify why the fl is a factor on these small electronic flahses... when you change the fl setting on the flash, (or on thelens, which autmaticlly changes it on the flash in TTL mode) it increases or decreases the angle of coverage to accomodate the part of the scene that lens will see on film. So at the same subject distance, the flash set at 35mm fl, will yield a 35 GN, while at 105mm fl setting, it will produce a GN of 54... meaning more light can be directed at the subject because the angle of coverage has been reduced.

-- Bill Glickman (bglick@pclv.com), April 08, 2000.

Bill you are asking about the coverage of a variable focus flash, which is necessarily independent of the focal length of the lens on the camera. Camera lens focal length plays no role in determining how the GN of your flash.

-- Ellis Vener (evphoto@insync.net), April 10, 2000.

Ellis, I realize the lens setting is independent of the fl setting on the flash.... however, the fl setting on the flash determines the GN. When a flash is advertised to be 100 GN, that assumes a certain setting fl setting (or zoom) on the flash. As the fl setting becomes higher, the guide number increases because light is directed in a smaller area of coverage. Most flash manuals give the different guide numbers for each of the fl settings on the flash... for example CAnnon 540's go from 24mm - 105mm. But what I wanted to know, is how do I calc. what the GN would be at 300mm. The manual does not show this because the flash does not go past a 105mm setting. Am I explaining it better now?

-- Bill Glickman (bglick@pclv.com), April 10, 2000.

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