Guide No. of flash on Elphs??

[fredpb]

I have an SD100. Anyone know the approximate guide numbers for the flash on this type of camera? On A75? On S1 IS????

[Paul Engen]

You can find it if you have a manual mode on your camera. Then you put the flash at max burst. Then you find the distance between the flash and the object where the object is correctly exposed. Then you can calculate what guide number you have in your flash. (I'm not shure if I remember how to do that, but I think you devide the distance with the f.stop or the other way around. You will find this on the Internet or in a proper text-book.)

You could also move away from the object using the auto-flash mode, and when you see the pictures start to be underexposed you know you have full bursts. Use the last correct exposure made ("on your way") and do the above calculation. (Using the auto-mode it will help if the object stands in the same focal plane as the surroundings, and that the lightness of the motive (and surroundings) have an average tonal caracter. This will not be a problem using a manual flash mode.)

[Paul Engen]

Well, I had to check it up... :-) Guide numbers on flashes (as far as I know) allways referes to using a 100 ISO film. With that in mind you can calculate the guide number using the following formula:

guide number = f.stop x distance in meters

You can also turn around on this formula to find the f.stop or distance (when you know the guide number). I hope this will solve your problem! :-)

(You should know that the ISO reference in some Canon cameras is not quite accurate. That makes no problems what so ever in a compact camera (picture quality). My former camera's (Powershot A80) ISO 50 was closer to ISO 100. I think I read that in a www.dpreview.com (http://www.dpreview.com/) test of this camera. If that is the case with your camera as well, it would prove impossible to find the exact guide number, exept if you can get hold of a proper flash-measure tool.)

[Jon]

GN may be given in metres or in feet, depending on where you are, although with Canon metres are more common. Read the fine print, though, before using GN to calculate aperture/distance so you're sure.