Originally posted by a5m it difficult for me to think of a reason why Av is preferred over P mode.
P, or Program, allows the camera to chose both the aperture and shutter speed. It is a safe mode for a safe exposure, but the photographer is essentially pointing, maybe focusing, and shooting.
With Av, the photographer that wants to do more than point, focus, and shoot, is making an intentional decision on what they want to do with their Aperture Value (a.k.a. f/stop). On a kit lens, if I want to shoot a portrait at 55mm, setting the aperture at f/4 (the lowest number=largest aperture) will give me shallow depth-of-field with my subject in focus and the background out of focus with maybe nice bokeh. Secondary advantages of this include the ability to shoot in lower light, lower ISO, faster shutter speeds. (In Av, the camera will automatically set your shutter speed for a normal exposure based on you ISO and aperture settings)
If, however, I want and need great depth of field, I can set the aperture to f/16 or f/22, and the camera will automatically lower my shutter speed for a normal exposure. OR I don't give a rip about my depth of field; if it is too shallow, it may not be very sharp, could be issues with chromatic aberration, and I could miss critical focus; if it is too great, I'll suffer more diffraction and less sharpness. So with Av and setting it to typically about 2EVs from the largest aperture (around f/4-5.6 on f/2-2.8 lens or around f/8-f/9.5 on an f/3.5-5.6 lens) I will get the sharpest IQ, least CA, etc, of the lens at that aperture.
Of course, I shouldn't ignore what is happening with my shutter speed, but it is secondary when I'm using Av.
I suspect most folks using P are simply not aware of all the nuances of high and low ISO, aperture, and shutter speeds, and just want an average exposure that wasn't messed up because when they tried to use M, they made too many mistakes. With Av your control dial is your way of intentionally using a specific f/stop and the camera will set the SS for you. Makes sense?