Historically, I have used the old rule of thumb, that to prevent blurry images, shutter speed should be at least 1/(lens_focal_length) or in the case of a cropped sensor, 1/(1.6xlens_focal_length).  Thus, if we were to follow the rule, a Canon 50D with a 200mm lens attached should minimally have a shutter speed of 1/320 second.

There are times when I think I could make use of Auto ISO in combination with AV priority on my Canon 50D. To able to do that effectively, I need to understand how the camera makes its decisions on selecting the shutter speed and ISO setting. For example, does it take into account the 1.6 crop factor of the camera or does it simply use 1/(focal_length)? Does it take into account whether a stabilized lens is attached?  Thanks for any help understanding how this works.