I intentionally phrased the question title a bit provocatively. What I mean is this: Aperture priority mode (Av) means that I select an aperture, and the camera selects the corresponding shutter speed. Shutter priority mode (Tv) means that I select a shutter speed, and the camera selects the corresponding aperture. Right?
So, for a given static scene, by me selecting any given aperture in Av, the CPU sets a corresponding shutter speed. And by me selecting any given shutter speed in Tv, the CPU sets a corresponding aperture.
Can someone please explain to me how the two things are any different?
Indeed, when I try it out (I have two different EOS Cameras, both are 35mm film SLRs: EOS Rebel Ti and EOS 620), I get either exactly the same, or very similar, shutter speed / aperture value combinations, regardless of whether I'm in Av or Tv (for the same static scene on a tripod, with static light). I can even set Av and "pretend" I'm in Tv, and the other way 'round.
So why are there two different modes or what's the difference between them? And isn't Program Shift, too, essentially the same for all practical intents and purposes?
Answer
Correct. As a matter of fact, on most cameras Program Shift is the same too by your logic.
These modes, including Program, are designed to give you the same exposure which is why the results are the same since Aperture and Shutter-Speed are inversely related given a fixed ISO.
The difference is what you decide on. Only you can decide if you would like a photo to have a particular depth-of-field or freeze/blur certain motion. You could attempt and guess what shutter-speed will give you the aperture with the depth-of-field you want, but is it not easier to simply choose the aperture?
You may note this in the behavior of the camera. In Aperture-Priority, the camera displays the chosen aperture at all times and the shutter-speed appears when the camera meters, usually on the half-press of by pressing AE-L (Oddly marked by * on Canon).
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