Is it possible to change the assignment of buttons/wheels on 600D/ T3i to command different operations other than the standard default settings. Specifically can the finger wheel on front be programmed to control the aperture exclusively as apposed to shutter speed?
I don't believe so. I have the T3i and it requires you to press the AV button whilst dialing the finger wheel in video mode to adjust aperture. Once you get the hang of it, it's pretty easy to do one handed.
Magic Lantern allows some control adjustments for the 3x zoom function, but not aperture that I know of.
I had a 550D and it looks like the controls are the same. It takes getting used to but in a hurry in bright light where you cant see the screen a few times I didn't have the button held tight in the back and changed the shutter speed instead.
I found this post on another site and it looks like there are work arounds to lock the shutter speed. have a look and see if this make sense and is possible to do.
Thanks/
http://www.magiclantern.fm/forum/index.php?topic=3459.0
Quote from post
I find that when I'm in a situation where there is a wide variation of lighting conditions (eg. moving around a room, near windows, etc) and I need to constantly be altering the exposure settings to account for the different conditions, that using the key shortcuts can be a pain. This is made worse by the fact that my viewfinder loupe further restricts fast/easy access to the arrow keys.
In that type of situation, where generally I've a preset and fixed shutter speed that I don't want to alter, being able to instantly scroll through aperture stops with the main wheel, without having any pre-selection and without having to look away from the viewfinder, would be very fast/ideal.
I can select and hold 'AV button' and turn the main dial - and then I get what I want, but again this is not as good as a direct assignment of the main wheel to Av.