Infinity focus and beyond

Do you ever test or familiarize yourself with auto-focus accuracy at the infinity mark?

If the focus scale on your lens stops at, lets say 40 feet, and your subject is constantly at say 100 feet or more, like planes at an air show for example, would you still use auto-focus or would you turn that off and manually set the lens to a focus point? I suppose you could turn on Live View, magnify to 5x or 10x, and focus well on a distant static subject to dial in the focus.


What would you do in this situation?

Static or very slow moving subjects, I prefer to use LV focusing anyway. Anything moving however, I rely on AF. It's way faster than I can ever be.

As someone who likes to shoot at airshows all of the time I have to say that I'm going to let the phase detect AF do it's thing.

Back in the 70's when I first started out it was all MF, and usually the end stop of the focus travel matched up with the infinity focus of the lens. This is good, because as you say it's normally then OK to just have the ring turned fully to the stop. This is not true for AF lenses though. Because the AF lens is powered by a motor it is given a bit of room for the drive system to overshoot. If you didn't do this the drive would be constantly hitting the end stop, which will quickly cause damage to the AF mechanism. So it's actually quite hard to set the focus manually for something as dynamic as an airshow, since you can't just put it against the stop. Move the lens past the point of infinity focus and you just end up with a blurred image.

I have older equipment, a 50D, and I have it set up for Back Button Focus only, and have it on continuous focus. I also set it to not drive if it can't find a target. Really useful as you don't want something like my Sigma 150-600 suddenly deciding it needs to run the focus back to MFD in the middle of a pass. For airshows I usually use the all points option. It starts out trying to pick up the subject on the centre AF point, but then will track to the other points as necessary. By being on the back button I can control the AF independently, and it's usually both faster and more accurate than I ever was with a fully MF system.

Photo from BigAl007's gallery.

Photo from BigAl007's gallery.

Photo from BigAl007's gallery.