> Please can someone explain what Guide numbers are and what they do? > > I use flash (on auto and on manual) all the time and have varying degrees of > success...getting better at it I think, but have no idea what guide numbers > are! A guide number is a value determined by using a flash meter to test the full output of a flash on manual. from the distance to the flashmeter and the f stop indicated, you can determine the GN (guide number). The method of doing your own test is better than trusting manufacturers specifications, as individual units vary. So say we set up a flash unit at 4 meters (measured!) and set the flash to full power, fire it at the flash meter and note it says 'f 11' - we then multiply the distance by the f stop to give us the Guide Number of 44 .. or for ease - 45 (there's barely a sniff between 44 and 45) Congratulations! you've just determined your first guide number :) from this and knowing photography basics we can extrapolate from the guide number either in distance, or f stop to work out the amount of exposure we need for the amount of light produced. backtracking to basics for a moment - in photography, think in stops as everything is halving or doubling :). f stop progression (halving the light each time) 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90, 128 iso: 50, 100, 200, 400, 800, 1600 back to the GN. When a GN is stated as say "GN 45 at 100 ISO" this also means "GN 60 at 200 ISO" or "GN 90 at 400 ISO" - the amount of light doesn't change of course, but as the film/sensor increases sensitivity by 1 stop (doubles), the aperture must be closed down a stop to compensate for the same amount of light to render the same correct exposure. So knowing you have a flash with a GN of 45, you can walk into a room, note the folks you are to shoot are 2m from the flash head - make a quick calculation (100 ISO, GN 45 - 45 divided by 2 = 22ish) set the aperture to f22 and shoot. hang on, the uncle's up on the roof, plastered, waving a bottle of champagne and you *need* a shot of him before he ends up in a cast ! - he's 6 meters away (45/6 = 7.5-call-it-f8) set the aperture and snap before he hits the concrete. done. Tricky one that actually! An auto flash would have been fooled by the abyss of night sky behind him and would have cranked up to full power waiting to get some reflection of the old guy in a black suit and he would have been blown out to white! GN's work :) this making sense? While you were working out the GN for full power back at the start, you may as well also work out the GN for the other manual settings at the same time. If you use a diffuser, mark the GN of the flash on that too for the different power settings.. What you'll end up with is a few numbers scrawled on the flash unit FP = GN 45 1/2 = 22 1/8 = 11 1/16 = 5.6 or somesuch and your diffuser will have something like this scratched onto the side FP = GN 22 on Metz 45 1/2 = 11 1/4 = 5.6 etc.. It's also handy if you know the GN and you're shooting strobe as the individual pulses add up! Say I use my quantum head with a low power setting and the GN is a mere 5.6 (f 5.6 at 1m OR f1 at 5.6 or anything in between) say I let loose 20 pulses remember the halving/doubling stops thing - 1, 2, 4, 8, 16, 32 etc? well 20 pulses is half way ish between 16 and 32, and one pulse is GN 5.6 so 2 pulses is one stop more (GN=8), 4 is another stop (GN=11) 8 is GN=16 so 20 is co-incidentally roughly GN 20 (and that example is pure co-incidence!) so my subject is 3 meters away - 20 divided by 3 is f 6.7 (the half stop between 5.6 and 8) Now. Everything that remains stationary will at the 3m get the correct illumination, but anything that moves will get 5 1/2 stops less light - so this has to be considered. it';s really kinda easy once you get your head around it :) karl
