P mode with program shift vs Av and Tv modes?

[always_learning]

I've been shooting with my new 10D for about a month now, and I have a question. When using P mode, I can adjust the aperture and exposure time using the two rotary controls. I often do this to dial in a particular aperture, for example. Given this capability, why would I use the Av and Tv modes, which seemingly do the same thing?

[PeterS45]

I suppose you would use the Tv or Av when you don't want any changes in the light on the last moment affect the shutter speed or aperture you want to use.
When you're in Tv a change in the available light will only affect the aperture, while in P it will affect both and you'll have to start turning the wheel again.

[richpix]

I've been shooting with my new 10D for about a month now, and I have a question. When using P mode, I can adjust the aperture and exposure time using the two rotary controls. I often do this to dial in a particular aperture, for example. Given this capability, why would I use the Av and Tv modes, which seemingly do the same thing?

The adjustment in P mode only lasts for one frame or one burst of frames. When you go to press the shutter again it will revert to what the camera thinks is the best combination of shutter and aperture.

If you know you want a certain aperture or shutter setting, it's easier to to use Av or Tv. It's too easy for me to forget I'm in P mode, so I never use it.

[Guillermo Freige]

Also flash works always as fiil flash in Tv or Av mode. In P it works as fill or as main source depending the situation and light levels, so it's easier to get a particular effect using Tv or Av combined win Exp.compensation and Flash Exp.compensation to control the foreground and background exposures.