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02-18-2010, 04:44 AM   #1
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Calculating with guide numbers questions.

I have several questions regarding OCF.

Let's say I use a Metz 48 AF-1, it has a GN of 158. So does that mean on full power it will always be 158, and when I reduce it's power to 1/2 it will be halved to 79?

Also in P-TTL or TTL mode, you have absolutely no way of knowing which GN it is in because it automatically sets the flash based off camera settings for you, correct? So to calculate everything, you need to set flash to manual only?

So let's say I set my GN to 79 to conserve battery power, I would have to do 79 divided by how many feet my subject is away to get the needed f-stop?

So my subject is 15 feet away, I would do 79 / 15 = 5.26. With the number 5.26, I would use a f-stop of f/5.6 since it's closest?

Not sure if this correct, but I believe it is. I'm confused here, help me out! Thanks.

EDIT: What if there are more than one flash? Would I only have to calculate whichever one has the highest GN?


Last edited by LeDave; 02-18-2010 at 04:51 AM.
02-18-2010, 08:38 AM   #2
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don't forget ISO

Guide numbers are based upon ISO 100

if youare at a different ISO you need to adjust the guide number as well.

Regarding Guide numbers and calculating F stop and distance range, the formula is

Guide Number = Fstop * distance

so yes, you are correct, if you cut the flash to 1/2 power, you have 2 options, re-calculate your distance or open the lens up one stop.

With respect to P-TTL or TTL flash metering, the only thing you need to actually calculate is the maximum or minimum flash distances, this will insure you are within the exposure range that the flash can operate within. Otherwise you are correct, the camera controls everything,

If there is more than one flash you would need to look at the contriubution from each, but it is not that simple because the guide number and distance calculations assume that the flash to subject distance is equal to the camera to subject distance. with multiple flashes this is almost always NOT the case. For that example you should assume that for each flash you take for distance (flash to subject + subject to camera ) /2
02-18-2010, 08:39 PM   #3
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Thanks louwel gouge, pretty much answered it.
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