s90 Flash Questions

[dalefield]

Has Canon published a Guide Number for the s90? The only numbers I've found published are 50cm - 6.5m (1.6 - 21 ft) for 28mm and corresponding telephoto numbers. I've also read that these numbers assume the camera is in Auto ISO mode. I can actually get much better reach with the flash at full power, however, at f2 if crank the ISO up, so I'm not sure these numbers reflect Auto ISO, however, 6.5m/21ft seems about right for full power @ F2 & ISO100...so, I've been assuming the Guide Number is around 42ish, but I'd like to know if there's a confirmable number.

Also, when setting flash power manually, the three settings (full, 2/3, and 1/3) do not seem to be in even thirds. Full power is as described in the paragraph above. 2/3 seems to be about a full stop, or possibly even slightly lower than full, which would indicate that it is actually about 1/2 power. Finally, the 1/3 setting seems to be useful only for Macro shots or triggering slaves without adding any direct illumination (which I'm very happy with and is actually how I've been usinng it). Does anyone know the actual power settings of the three stops?

Thanks!

[dalefield]

Has Canon published a Guide Number for the s90? The only numbers I've found published are 50cm - 6.5m (1.6 - 21 ft) for 28mm and corresponding telephoto numbers. I've also read that these numbers assume the camera is in Auto ISO mode. I can actually get much better reach with the flash at full power, however, at f2 if crank the ISO up, so I'm not sure these numbers reflect Auto ISO, however, 6.5m/21ft seems about right for full power @ F2 & ISO100...so, I've been assuming the Guide Number is around 42ish, but I'd like to know if there's a confirmable number.

Also, when setting flash power manually, the three settings (full, 2/3, and 1/3) do not seem to be in even thirds. Full power is as described in the paragraph above. 2/3 seems to be about a full stop, or possibly even slightly lower than full, which would indicate that it is actually about 1/2 power. Finally, the 1/3 setting seems to be useful only for Macro shots or triggering slaves without adding any direct illumination (which I'm very happy with and is actually how I've been usinng it). Does anyone know the actual power settings of the three stops?

Thanks!

Any chance someone out there has a flash meter and an s90 who could take a few quick measurements? I'm especially curious about the manual power adjustments.

Thanks again!

[dalefield]

Any chance someone out there has a flash meter and an s90 who could take a few quick measurements? I'm especially curious about the manual power adjustments.

Thanks again!

OK...I see a bunch of folks have read the post. Since others may be interested in the answer, I'll take a shot at answering my own question. Take my conclusions with a grain of salt...my methodology was as follows: I set up a gray card a set distance away and took a picture at maximum flash that showed proper exposure using the histogram. Then, I took pictures of the same gray card at the 2/3 and 1/3 setting and adjusted ISO until I got (as close as possible) identical histograms. It's not a perfect method, but it's close enough, especially considering I don't care so much about the technical specs as knowing how to make adjustments to get the shots I want.

So, the answer is: From Full Power @ ISO 100 is almost exactly equal to 2/3 Flash @ ISO 320, which is approximately equal to 1/3 Flash @ ISO 3200 & opening the aperture one stop as well.

This translates to 1.67 stops @ 2/3 Flash and 6 stops @ 1/3 Flash. The 1.67 and 6 stops are the numbers that are really important to me in real-world shooting, and if I'd known the answer to the power question, I would have used it to calculate these answers anyway. Still, since I've got these, I worked backwards to get the following - if I recall my flash math correctly. 2 full stops should be 1/4 Power, so the 2/3 setting is slightly more than 1/4 Power, but significantly less than 1/2 Power. 6 stops should be 1/64 Power.

I don't know if this is accurate...again, if someone's got a flash meter, it would be nice to be sure...but in the real world (actually, a gray card in my bedroom, but at least it was consistent for comparison), I think 1.67 stops and 6 stops is a close enough answer to have confidence making adjustments on-the-fly. If anyone tries to duplicate this and gets different results, please post and I'll have another shot at mine. Otherwise, I hope this helps someone.