Canon XSi with Sigma EF-530 DG Super

[mikewithxsi]

I have a canon XSi with a sigma EF-530 DG Super.

If I set the camera to ful auto with the flash in ETTL mode the camera sets the aperture as high as possible (F3.5 @ 18mm) and the shutter speed to 1/60. I assume it's adjusting the flash output appropriately, although it tends to overexpose slightly. I found a flash compensation of about -2/3 works best and I force it to iso 100 or 200 instead of the auto mode iso 400.

What I don't understand is if I set the camera to aperature priority mode on the same shot, the shutter speed will then be slowed down significantly. (i.e. right now as I write this where F3.5 and 1/60 was a bit overexposed, at aperature priority mode it tries to drive the shutter speed to 1/4 second. The flash power is reducd to compensate, but why? Is this normal? How can I get the same flash power as auto mode in aperature priority.

If I switch to allow the shutter speed take priority and set to a speed faster than 1/4 sec the F3.5 will blink and the shot will be underexposed. Again, the flash outputs less light than the auto mode where 1/60 & F3.5 overexposed.

Any help would be appreciated.

Thanks

 

[o hey tyler]

What you need to understand when using flash is that your shutter speed doesn't control your flash output power, or the exposure of your flash. This is determined by your aperture, which is how much light is let into the lens through a diaphragm. The flash burst is so short and bright, that it happens during the time that your shutter is rolling (up to the x-sync speed. 1/200s on your camera)

Shutter speed WILL control the amount of ambient light that is exposed for (I.E. overhead lights, the sun, anything continuous light source). ISO will amplify both of them, so I suggest having your camera on your native ISO setting of 100. Try shooting indoors at 1/160s @ f/5.6 ISO 100 with the flash on ETTL. You'll need to have your camera on fully manual, but you should be able to take flash photos indoors and not have to worry about camera shake or motion blur.

 

[mikewithxsi]

I do get good results in full manual mode, but I still don't understand why I can't get the same results in aperture priority mdoe or shutter speed priority mode.
How is the light meetering actually done? Does the camera send the exposure settings to the flash and it controls the flash output or does the camera send the expected flash output to the camera? When is the meetering done, durng the shot or before?
Thanks