7D Shutter Speed Different in Av and P

[nathansym]

Hello all.

I read DPS every day (RSS) but this is my first post.

I have a question.
I was playing with my camera while reading the DPS portraits book.
I noticed that while in Av mode I was getting very long shutter speeds even when I was using flash.
When I switched to P mode and let the camera decide both(with flash on), i noticed that it was selecting the same Aperture but a much short shutter speed (from 4" to 1/60).
Also the ISO is set to 100 when in Av and P so that isnt changing.


I also noticed that the P mode was sometimes not firing the AF assist beam (but still coming up with 1/60!) but when in Av mode it seems to always fire.

Both the flash exposures are set at 0.

Can someone please explain what may be going on!?!?

[nathansym]

Got it ... flash sync speed in Av mode

[inkista]

Yes, and no. But basically yes.

This article explains it pretty clearly.

Canon made the assumption that if you're in Av/Tv, that you'll mainly want to use the flash for fill. So, the shutter speed/aperture is set pretty much the same as if you were doing an ambient-light exposure, with only enough light coming from the flash to fill in shadows and "even up" the subject.

In "P" mode, the assumption is that the flash is your main source of illumination.

You can make Av behave like P by setting the custom function that forces a specific shutter speed in Av mode when using the flash.

Ideally, you should be shooting Manual mode, so you have control over the iso/aperture/shutter speed, and you can specifically choose how you want to balance the flash against the ambient.