Quote Originally Posted by Asylum Steve
Good point about the diopter, Michael. It's possible...

I don't want to sound too old, but I never used auto focus with film, never wore my glasses when I shot, and could always tell when the shot was in focus manually. It was mostly due to the great Canon focusing screens with the F-1 and EOS film cameras.

I had the split prism center circle and the ground glass edges, and it was flawless. When the top and bottom halves matched up, it was in focus...

Now, I shoot all the time with my DSLRs in auto focus, and it works very well. I've even gotten good at quickly adjusting the focus point depending on what part of the frame my main subject is.

With the dim light, I couldn't get the auto focus to work. It kept focusing in and out, so I switched to manual.

With the manual focus on the 5D, you should still get a beep and see a focus point light up in the viewfinder when it focuses on a specific spot, but I don't remember it doing that.

Basically, I got sloppy. Had the damn lens wide open probably at too slow a shutter speed for the heavy camera with the zoom. I may have not held it tight enough. Anyway, my margin of error was tiny, and it obviously wasn't enough...
Well I hate the view finder screens in the DSLRs. They don't have the goodies for manual focusing, the micro prisms and the split prism center circle. Makes it very hard to use them in manual focus, or full manual mode with older non AF lenses.