Canon Custom Function C.Fn IV -1 Shutter button/AF-ON button

It's been a year since I was a Canon shooter, and I don't think Tim has used the Mark II long enough to understand the usefulness of the AF-ON button.

I'd agree with what you said about #2 for sports. #1 is useful with wildlife photography, mostly birds, and long lenses, where if the camera looses focus, you want it to stop trying to focus on the sky so you don't go too far off course. I have a lens that limits the focus range for the same reason.

#4 serves no point unless you're always hitting the button by accident.

#3 looks like the old switch AE-lock and AF. I *loved* this mode as it allowed you to actually seperate focusing from taking the picture. Basically it gives you the ability to take a picture out of focus if you wanted to because the shutter release is still the same button, but you can "pump" the AF with the AF-on button. So if you're taking pictures where something may get in the way of what your main subject is, your focus will remain at the same place until you tell it to refocus, allowing you maximum shutter response.

I'm sure how much of the Edsel's brains they put in the 40/50D and 5DMark II, but on the Edsel, because of the "object gets in the focus line between you and the subject" there was a custom function to program the amt of time the camera would actually ignore something like that. Never got that to work. (Canon told us that the problem wasn't with the camera, it was that we didn't set that feature right. Okay. Riiiiiiiiiiiiiiiiiiight. Grass will always take priority over the subject under the center focus point.)