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Guide numbers, distance, f-stops, flash power and zoom setting on the fly help

Pete S. , Feb 21, 2009; 09:22 p.m.

I'm trying to get my head wrapped around the idea of using guide numbers to get a good starting point when using small flashes. The thing is that I'm trying to find a way to do it quickly, in a few seconds tops, and my head is starting to explode.

Here is the scenario. I usually have my ambient exposure down, let's say f/2.8, 1/30s, ISO400 and this is fixed. I also have the distance set, let's say 2m. The flash has a guidenumber of approx 30m at 35mm zoom and ISO100 but it's set at 14mm so I have to account for that. Now what power do I need to set the flash to (everything from 1/1 down to 1/64 in 1/3rd stop increments is possible)?


Dan Ferrel , Feb 22, 2009; 12:59 a.m.

So your either using a SB-600 or a 430EX right?

And that guide number of 30m is only for ISO 100, now you have to account for two more stops of light since your at ISO 400. But the 14mm means your now at... wait, there has to be a better way.

http--www.night-ray.com-PhotoCheatSheet.pdf Print a cheat sheet and get back to shooting. Pages 6 and 7 are for the 430 and sb600.

Why are you at 14mm? If it's not because you have a lens that's wider than 24mm then your wasting light. The zoom is supposed to correspond with the angle of view for a lens at the same focal length. i.e. a 35mm lens has the same angle of view that 35mm zoom on your flash puts out, with little light wasted.

Pete S. , Feb 22, 2009; 07:59 a.m.

Thanks Dan. But just using the chart you linked to, I still can't get the answer to my question. But your reply led me to a another good solution which is to think completely in stops instead.

So at a distance of 1m using f/1 and ISO 100 the flash will overexpose 10 stops (GN 32m).

Using f/2.8 (-3stop), ISO 400 (+2 stops) means I loose one stop. Using 14mm zoom instead of 35mm means I loose a little more than 2 stops more. Shooting at 2m instead of 1m means another 2 stops. So all in all I loose 1+2+2=5 stops. Since the flash will deliver 10 stops and I loose 5, I have to turn the flash power down 5 stops to get the right exposure. 5 stops is 1/32 power.

You are right about the flash. I use modified SB600s, my favorites for on camera flash. I also have a number of other Nikon speedlights like the SB24 which all have about 1/2 stop more juice.

BTW, the reason for 14mm is that I know from experience that I need drop a lot of power and the additional coverage of the flash means I will have less contrast from light reflecting of nearby objects. So I usually shoot the flash a little wider than the focal length when using it straight on. When bouncing I go tighter, usually 50mm or 85mm even if I'm at 35mm FL.

Dan Ferrel , Feb 22, 2009; 02:52 p.m.

Well at least the chart gives you something to go off of. It breaks it down to ISO, zoom, and aperture, and is a solid chart to work from. Knowing that you have a gn 43 flash (or what ever) doesn't give you enough info when your trying to factor in the zoom rating. 43m is only for the max zoom on the flash, which I find myself using very little.

If your trying to lose power on the flash due to the limitations of the flash's power level not going low enough, why don't you drop your ISO? You'd get cleaner results with less noise as long as you have enough flash power to keep up. If your trying to match the sun, a low ISO is better in my opinion so that you can utilize the sync speed to your advantage. The sunny 16 rule shows f/16 @ 1/ISO for shutter speed. You knew that I'm sure, but factor in your max x-sync and things become more apparent. ISO 100, f/16, 1/200th means you just dropped the suns power by 1 stop, so you can actually shoot at f/11 for the same exposure from the sun while letting your strobes catch up a stop. If your shooting at ISO 200, you don't have that much room to work with. Of course this is all dependent of what your trying to achieve and what environment that your trying to achieve it in.

You can also use ND filters on your flash if they are still too much and those are labeled by the number of stops they cut (or you could just use a lower powered flash).

When bouncing I usually don't bother setting the zoom, unless it's really high. I don't usually have the time with what I do. But since it's a bounce, the light's going to spread so even if you set the zoom to something like 105mm you should still get pretty even coverage, but again, I don't usually bother.

That chart also shows a few other flash's as well as some studio strobe too.

But back to your calculations. The chart clearly shows @ f/2.8, ISO 400, 14mm zoom you have a guide number of 8m. To drop to 2m follow the chart down and you have 4 stops you need to lose, not 5. Just find your ISO, f-stop, and zoom and go from there. 4 lines lower is 2m, so lose 4 stops (1/16th power) no real calculations involved. Or if you just dropped your ISO to 100, you'd be able to shoot at 1/2 power and have that lower ISO smoothness.

Nadine Ohara - SF Bay Area/CA , Feb 22, 2009; 06:02 p.m.

I just use the flash's calculator or make a cheat sheet and tape it to the flash (the most used combinations and settings). Any transposing can be done pretty fast. I don't understand why you would want to do something like this.

Pete S. , Feb 22, 2009; 07:22 p.m.

Dan, you're right about the cheat sheet, it's a good start but I would like to manage without it if possible. It's just my personal preference to be able to calculate it in my head instead of having to whip out a table. Then to be able to do it fast is just a matter of practice.

When I calculated the example I approximated a loss of two stops going from a zoom of 35mm to 14mm because 14mm is roughly twice the spread hence 4 times the area, or two stops loss. Now looking at the numbers in the SB600 manual it's actually just a tiny bit more than two stops (from GN30m to GN14m). So when redoing my calculations on paper I found out my fast approximation was only 1/3rd of a stop off, the correct number would be to drop the flash 4 and 2/3rd of a stop. The cheat sheet is a combination of numbers from the SB600 and 430EX and you get into rounding problems as close as 2m so it's not 100% accurate here.

And I just tried this exact combination and it turns out that with the flash power set to 1/32+1/3 the exposure is perfect , 1/32 is fine as well but 1/16 is too hot and blows the whites.

Anyway, I found a solution that I'm happy with and that is to think in stops only. Horses for courses I guess :-) Thanks for your help.

Dan Ferrel , Feb 22, 2009; 10:44 p.m.

Hey, if reading that made something click I guess that's gonna work. I have a 430EX (Canon shooter) and without that cheatsheet I'd have no clue what the GN for different zooms were. It's not covered in the manual.

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