Flash Question

Hi

Just a quick probably silly question about Flash. I have read the excellent Flash documentation mentioned on this site and it was a great help.

I just have one remaining niggle.

If I shoot in M mode outside and ISO is set to say 100 I will be using Fill Flash if the light is good and if I feel I need flash.

The problem I have is when I go inside. If I still use M mode and I set it to say F8 1/60th I realise that I am setting the settings to expose the ambient light and the flash will do the rest.

My question is do I change the ISO as well as I would do if I was not using flash and went from outdoors to indoors?

Sorry if this is silly or technically incorrect, but I think I am thinking about this too much and therefore getting very confused.

I'm not exactly sure what you're asking, but the ISO setting works the same, whether you use flash or not. The exposure of the photo is determined by three things: aperture, shutter speed and ISO setting.

Canon's E-TTL flash system takes the ISO setting into account, so if you turn up the ISO, the flash will give off less light to make sure the exposure will still be correct.

Ok so if I change the ISO from 100 to 400 I will get a more powerful Flash at ISO 400 than I will at 100

As I understand what Jesper said, the opposite will be true: at ISO400 you'll get LESS flash than at ISO100 because the camera needs less light to shoot a picture at ISO400.

The best way to get your answer, and to find out what will work best for your tastes, is simply to try it.

Ask a long-suffering friend to come be a test model and shoot some pictures outside with fill flash, then go indoors and do the same thing and see what happens.

Nothing like first-hand experience to help a person really learn what needs to be done.

But don't try it on a critical shooting mission (i.e. your daughter's birthday, the arrival of a new puppy, whatever) until you've tested it out, even what anybody on this forum says: test it all out when it doesn't matter.

You can use your ISO to enable you to get a larger depth of field (higher f stop) or increase your distance from the subject and still maintain flash coverage. When you increase your ISO to 200, the flash coverage is increased by a factor of 1.4. If you increase the ISO to 400, it increases by a factor of 2.
At ISO 100 the Distance a flash can travel is Guide Number / f stop.
Scott

That is assuming that this is Canon equipment and Canon Guide Numbers, which are in meters. I've seen other manufacturers who use Guide Numbers in feet, so the number looks roughly three times greater, even though it isn't more light.

---Bob Gross---

Bob,
The formula holds true regardless of who makes the equipment and regardless of whether you use feet or meters.The multiplcation factors are the same as well. If the Guide Number is in feet the distance will be in feet. If it's in meters, the distance wil be in meters.
Scott

Sure, the formula holds true, but my point was that some companies create their Guide Numbers in feet, whereas Canon does it in meters. So, a Canon with a Guide Number of 55 (meters) might be pretty good, and Brand X with a Guide Number of 55 (feet) might be crap.

---Bob Gross---

ok
s