| I would think the most likely time for this to happen would be where the light is marginal for the f-stop you are trying to use in conjunction with the aperture. If you are in Av mode, the aperture is the first thing to set. The camera will then decide upon shutter speed and ISO (unless you have the ISO locked to one value).
I think about the only way to determine what's going on is to get outside in some consistent light with plenty of exposure latitude, and then do a process of elimination. If you turn the camera off, it resets the aperture value, so that's what's allowing your change in that case. But if you keep the camera on, and are in Av mode, and set your desired aperture, it should stay at that aperture setting until you intentionally change it. Something else that may be happening if you're using the grip is that the ball of your hand may be hitting the green button while you're holding the camera. Then everything resets to what the camera considers the best value (at least that's how I remember it). When I first got the grip, I had that problem a lot - my hands are just the right size to create a "green button hazard" on the grip.
Good luck,
Barry |