Nearsighted
RFF-aholic
I have an old Vivitar Auto 251 flash I need to find out the Guide Number. I picked up an Olympus 35 EC which uses the guide number of the flash to pick the pproper shutter speed/fstop combination. Thanks
How to determine guide number
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Bob Helmond
Member
http://www.naturephotographers.net/articles0703/jm0703-1.html
The above link will take you to a well written explanation of guide numbers, as well as a method to compute the G.N. for a flash of unknown output.
However without a flash meter, it's still possible to compute. Set the flash/camera combo in manual mode on a tripod 10 feet from a test subject. Take a series of exposures, bracketing in half stops. Be sure to record the aperture used for each frame. Preferably use color transparency material for the film stock, since this will eliminate any "subjectivity" imposed in the translation of the negative to a print.
When the film comes back. An examination of the individual frames will indicate which is the correct exposure. The aperture (ƒ stop) used for that frame when multiplied by 10 will yield the Guide Number of that flash unit.
The above link will take you to a well written explanation of guide numbers, as well as a method to compute the G.N. for a flash of unknown output.
However without a flash meter, it's still possible to compute. Set the flash/camera combo in manual mode on a tripod 10 feet from a test subject. Take a series of exposures, bracketing in half stops. Be sure to record the aperture used for each frame. Preferably use color transparency material for the film stock, since this will eliminate any "subjectivity" imposed in the translation of the negative to a print.
When the film comes back. An examination of the individual frames will indicate which is the correct exposure. The aperture (ƒ stop) used for that frame when multiplied by 10 will yield the Guide Number of that flash unit.
payasam
a.k.a. Mukul Dube
Nothing to add except that reflections from walls and ceilings can affect exposure.
Dwig
Well-known
To start with, there isn't such a thing as "a guide number" for any flash. There are as many guide numbers as there are film speeds. There is one GN for each film speed.
There are two methods:
1. Accurate Results by Trial & Terror: Do Bob's test in a room typical of the type in which you will be shooting in terms of size and ceiling height. The results will be the guide number for the ISO of the film you used. For other ISOs you can calculate the GN by first calculating what change in f/stop would be needed to correct for the change in ISO (one stop faster film = one stop smaller aperture). All flashes have some light falloff toward the edges. It is generally best to judge the exposures at a point about 1/3rd out from the center, but be your own judge. This is the best approach as it finds the GN for the flash in its current condition.
2. Quick and Easy Approximation: Set the desired ISO on the flash's calculator dial as if you were using the flash in manual. Multiply any pair for f/stops and distances that pair up. Any pair you choose will yield the same answere within the accuracy of the calculator dial. This will find the GN at which Vivitar rated the flash when it was new and is likely somewhat higher than the flash's current performance.
