The logic behind IS? Too many questions.

This is not a gripe or complaint. I'm getting a 24-105 delivered and I want some advice to keep in the back of my head while shooting with it.

And before anyone says "IS won't stop moving action", we all already know this. Thanks for your help. Or, "With proper exposure, you can shoot with ISO 3200." Thanks again, but I really prefer shooting at 1250 and below.

I'm interested in how to adjust settings and/or complementing with a flash.

Here's a hypothetical situation.

Dimly lit room. Picture of someone sitting at a table smiling for you. You have a flash available.
AV mode at F4 on a 30D
ISO 800
Camera determines that a shutter speed of 1/20 will let in enough light for the standard exposure.

Some questions?:
Should I raise the ISO to 1600 to get a faster shutter speed?
Should I turn flash on and use in ETTL (still in AV mode), lower ISO and/or higher F-stop?
Should I turn flash on and use 1/8 power (still in AV mode)?
Should I turn flash on and go to M mode and let camera compensate automatically in ETTL?
Is using a flash with IS contradictory?

It would depend on the effect you are going for. In your example that sounds like a flash only picture where you are using the flash for the bulk of the light and the IS would accomplish nothing. If you were in a fill flash situation where the natural light would be providing a lot of the light then the IS could be helpful.

Flash and IS have little to do with each other really. IS is about handholding your lens and perhaps not running up the ISO.

I have found that people in candid poses will move a bit, so I have found that 1/60 sec will stop them if they are trying to be still, but slower than that can sometimes lead to subject movement.

gasrocks wrote in post #4986364
Flash and IS have little to do with each other really. IS is about handholding your lens and perhaps not running up the ISO.

Unless you are trying to balance the background exposure with flash for the subject.

No. 1/20 is more than 1 stop too slow for a shot of a living person, so another 1/3 or 2/3 stop will not be enough.

No, Av mode is terrible in general with flash. About the only time it works well is in good light when the flash is very strictly fill only.

Still no. Get out of Av for anything where the flash is more than a shadow fill.

Bingo. Try f/4, 1/60 and ISO640 with flash on. Go straight to 1/200 if you don't care what happens to the background and want to avoid WB issues.

No, is situations where you really want to preserve the background it can be useful. You example was 1/20, f/4 and ISO 1250 for an ambient exposure. You could shoot this same shot at 1/40, f/4 and ISO 640 with the flash. The IS will keep the background sharp, the flash will freeze the subject and the result will be good.

I still like to shoot with a flash in manual. 1/60 or faster as I don't have the steadiest hands and IS can only help so much. My 550EX has no problem providing the light I need, and I can keep the ISO down.

Manual, if you can. You can sync down to 1/200 if you want and let the flash do ETTL.

Or, with IS, you can handhold and get a little more light in by dragging the shutter and using the flash as fill. IMHO, this results in a far nicer looking image rather than a perfectly exposed subject and no background...reminds me of nightclub photos of teenagers taken by their friends.

L

Too many questions. You should buy a point and shoot, and drink orange soda.