Quote: In Av and Tv you can control the camera settings chosen. The camera metering will always try to give you a good combination but you can vary say the f stop beyond what's given or good for the exposure.
just to clarify this a little... In AV or TV you can select settings that the camera can't balance to obtain correct exposure. An example of this would be selecting a shutter speed that requires an aperture that your lens doesn't have. Although I've never tried this, on previous cameras it would just select the best it could do but the end result is a pic that's not exposed correctly. Another case where Av mode fails you is you select a small aperture (large number) and the resultant shutter speed is too slow to handhold.
Quote: In P mode (and I believe this is a true statement) you really can't override the camera's meter balancing out the f stop or speed based on your selection when you change the wheel (f stop or speed) because P mode is really green mode with user selection of f stop or speed.
Yep, you need to play with the exp comp to achieve a bias to the selected exposure (for whatever reason you decide). The P mode does allow you to do things that green mode won't like alter flash exp comp (I think that's one... I don't use green mode).
So I would explain Av, Tv and P modes like this: ("correct exposure" meaning the cameras idea of what's needed for a picture)
Av - You select Aperture and the camera selects a shutter speed to give correct exposure within it's capabilities (may not be possible resulting in incorrect exposure)
Tv - You select shutter speed and the camera selects aperture to give correct exposure within the lenses capabilities (may not be possible resulting in incorrect exposure)
P - The camera selects both aperture and shutter speed. The camera allows the user to override either the aperture or shutter speed and the camera will automatically change the other variable to try to maintain correct exposure. (I imagine you can cause incorrect exposure just like the other modes too by selecting settings that cause the other parameter to go out of range)
I leave my camera in P mode nearly all the time. I use the wheels (K10D and K7 so 2 wheels) to select either aperture or shutter speed to suit the subject if I don't like what the camera has selected. I use the exp comp to alter the exposure to purposely increase or decrease exposure based on a combination of the histogram, preview on LCD and my intent.
Nige.