Guide Number questions ??

I'm trying to figure out whether to buy an additional strobe for my Canon S30. I know I'll get better QUALITY light, but I started wondering whether I'll get more QUANTITY light with a lower priced strobe (DS50, YS 90or YS25) than my on camera flash. More quantity will equal smaller fstop, which equals better depth of field.

So, I started wondering what the guide number of my Canon is. I can not find that info anywhere. What I did find was the maximum flash range listed in feet. For ASA 100, the book said 13.1 feet for Wide and 8.2 feet for Telephoto. At first this difference surprised me, then I remembered the fstop changes as you zoom in.

I figured I'd get my guide number by reversing the formula: Fstop=GN divided by Distance. So if my max range is 13 feet, and that would be at widest fstop 2.8, then my GN would be: Feet X Fstop or 37. For wide angle. If they are right about max range dropping to 8 feet for Telephoto, then that guide number would 40.

So my questions for those who know are this:

Does anyone know where to find a published guide number for a Canon S30?

If not, does my math seem correct for figuring out the GN?

And how does this number (37-40) compare to the land based Guide Numbers for the lower priced Ikelite and Sea&Sea strobes?

Thanks,

GB

Set your camera at the maximum fstop and take a shot with your cameras internal flash. Then step back a foot and take another. Repeat.

Look at the photos - where does the flash start to look underexposed? Multiply your fstop by the distance and you have your land guide number for the camera's flash. Underwater guide number is usually a little less than half of that from what I gather.

Or you could just consult your owner's manual for the information...:-)

Cheers

James