How to set aperture and shutter speed when using ext flash?

Hello all,

Sorry if this is a question has been answered somewhere. I just could find the right solution.

I am using Sigma EF-500 DG ST and Digital Rebel. But I found when I use Av or Tv mode, the aperture and shuttle speed doesn't change according to the flash on/off. For example, if I set Aperture=8.0 in a relatively dark room, without using flash, the shuttle speed was automatically set to 0.3". But if I turned on my flash and did a preflash, the shuttle speed was still set to 0.3". I manually increase the shuttle speed in M mode, the output picture still was not too dark. It looks to me that the preflash only affect the flash itself but doesn't feedback the information to the camera. But when I use the internal flash, turning on and off flash does make shuttle speed different. Is usually in case of external flash, using the M mode the only way? If so, how do I know what aperture or shutter speed I should set? Taking multiple testing shoot doesn't look like a right solution to me...

Thanks.

When flash is your primary light source, M mode isn't the only way, but it's the best way.
Av and Tv modes aren't designed for this. They're designed to meter for ambient light.

If your flash shots are underexposed the first question that needs to be answered is whether it was because of a lack of flash power. I'm hoping the ST unit has a "ETTL" indicator on the rear display that flashes for a few seconds after each shot. If it flashes, that means the flash "thinks" it got the correct exposure and you need to increase the FEC. If it doesn't blink, that means you need to help it out with a wider aperture or higher ISO.

It might help you to read Flash Photography 101 (link in my signature).

Yea definitely check out Flash Photography 101 and it'll answer all the basic questions... that's what I did when I purchased my 430ex. ISO and aperture play a big part in overall (and background) lighting

try going to the custom functions in your camera ( C.Fn )and change the custom function #3 "Flash sync. speed in Av mode" , you can find more info on C.Fn on your camera manual.

hope this is what you are looking for.

Thank you all for the help.

Curtis, one quick question.

If the exposure is correct, why do I still have to change FEC?

Thanks,
Euro

The camera doesn't know what it's metering, it only measures light. So for example it will tend to overexpose a shot of a choir wearing black robes, and underexpose a shot of a polar bear in the snow. FEC is often necessary to account for the brightness of the colors in the image. Adjusting EC when metering ambient light is necessary for the same reasons.

I agree with Curtis. I prefer to not screw around with FET & ETTL. The guys who programmed ETTL don't know what I'm shooting? If you want to try full "M", take a look at this:

Fill light at sunset
Here's something that will help you figure light fall-off in your head