Av/Tv vs. program shift

Strange question - why would anyone ever use Av or Tv mode. Doesn't the program shift feature do exactly the same thing as both of these? Just shift the exposure until either the ap or shutter is where you want it. Maybe I'm missing something here...

But values of T or A will still vary with program shift. Tv you can pick a shutter speed and it will stay that speed until you change it.

-Pete

I will occasionally use Tv or Av instead of manual. Never program mode though. Program mode requires me to think too much about what the camera is doing and how to work around it. ie: expose - look at the result, shift it, whereas if the shutter or aperture is critical I can just set it once with Tv or Av and not keep playing around. Under constant lighting, I'll usually use manual so I don't have to worry about anything shifting around because the camera is fooled by what's in front of it at the moment,

First to clarify... Do you mean to say Program AE mode or are you refering to the Program Saftey Shift available in the 10D CF menu?

There are a number of reasons to use Av and Tv over the P mode. Absolute control of the depth of field or the ability to stop action being paramount. With Program mode you are at the mercy of the camera's A.I. to decide what it thinks you want, to me that is unacceptable. Don't even get me started on the idiot, I mean, Icon modes. There are also incredible reasons to use Av with flash photography. The ability to slow Sync and correctly balance ambient and flash exposure, especially at night, is simply amazing and I for one, couldn't live without it.


As for reasons to use the Program saftey shift, sometimes it is nessasary to CYA when you are shooting. If you set the shutter to 1/500 in Tv and that will result in underexposure then saftey shift kicking in and dropping the shutter to 1/25th and saving the image may be a very good thing. I am on the fence about using it. I have used it but I prefer to pre-meter the scene and plan from there.

I hope some of this made sense...

What I'm talking about is "Shifting the Program" (pg. 81 in the manual) where you push the shutter halfway and use the top dial to maintain a proper exposure while choosing either a desired ap or shutter speed. In Av and Tv, you choose the desired ap or shutter speed, and the other is set automatically, same way with the program shift, but you can choose to set either value without having to change modes. I guess the only reason I would use the Av & Tv modes is if I wanted a constant ap or shutter speed for an extended period, which rarely happens. I would much rather evaluate each shot on it's own and shift the values from P mode if I want something different.