About flash and aperture

[zeeman]

I've been working out with the Speedlites and still I'm trying to understand something:

How do I set the aperture that's appropriate for the flash?

I know that once I set the aperture that's appropriate for the flash, I can then freely adjust the shutter speed to lighten or darken the background. So the question is how can I find out that aperture? I might be missing the obvious but just don't get it.

Any ideas ? I don't have a light meter, just a Canon Speedlite 580EX II and a Canon Speedlite 430EX (plus the camera meter, or course).

[RyanMerrill]

Pick your aperture first. Find a good exposure without the flash that has the background properly exposed and then add your flash (chimping if you wish) and keep adjusting it until the flash looks the way you want it. I prefer to use the widest possible aperture so the flash doesn't have to work as hard.

[bobbyz]

If eTTL mode, your camera and flash will talk.

In manual mode, you set your camera to manual exposure, exposing for the background or ambient. Then you adjust flash power for your subject exposure. If you have a flash meter like L358, that will tell you what aperture to set the camera for given flash power.

[egordon99]

Yep, E-TTL determines how much flash power for your ISO/f-stop combination. So say you're at ISO200, f/8 and E-TTL determines you need 1/2 power to properly illuminate your subject. If you were to up the ISO to 400, you would then only need 1/4 power. Or, you could open up to f//5.6 and keep ISO at 200, and you'd need 1/4 power. OR go to f/5.6, ISO400 and you'd need 1/8 power. Basically, the higher the ISO and the wider the aperture, the less flash power you'd need for a given scene.

This also comes into play with HIGH ceilings. If you're trying to bounce off a high ceiling, you'll find that upping the ISO and opening up the aperture gives your flash much more "power/reach/effectiveness/etc..."

[zeeman]

Many thanks for the replies. Well its time to get those flashes into action.

I will first try to do it all manually to understand all these.

Once again thanks a lot !