Originally posted by Lowell Goudge Your ISO factors are a little off each doubling of ISO is 1 F stop. Guide number is multiplied by 1.414 (square root of 2) for each doubling of ISO, starting with 100, 200 and 400 are close should be 1, 1.414, 2) , 800 the multiplier should be 2.828, 1600 shoud be 4, 3200 should be 5.656

Lowell... thank you very much. I took a look at my AF160Sa and used the "slide rule" on the back to come up with the following scheme... The math is based on the calculations discussed above as further modified by Lowell's data. I did not go beyond 12 meters because I actually am not sure the flash would be effective past that distance. Perhaps someone can advise on this. I will test more. I did take one long shot in the house that seems to indicate that it may work at least that far. Perhaps the limitation really is the aperture. If so that would be expressed as:

Max Distance = GN / max aperture

Using that formula, the max distance at 3200 ASA would be 44/1.4 = 31 meters? Wow.

I did validate this chart somewhat at 400 ASA, and was pleased to find that it does seem to work. Method for validation was simple. Same as for using chart... focus on subject, look at focus window to determine distance to subject, set f-stop based on ISO and distance. Shoot.

If anyone cares to work though this and validate it a bit more, please feel free to modify this work to suggest a better tweak. I will do the same. Might take a little while, and I will also see how this hold up with wider angle versus telephoto.

Please tell me if you cannot see the following image:

If you can't see it, cut and paste the URL:

farm3.static.flickr.com/2296/2261117056_74774c80da.jpg?v=0

Respectfully,

Shane

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Last edited by woof; 02-12-2008 at 11:16 AM.
Reason: Fixed missing image.
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