100-400 Mark II Periodic Failure to Focus

I am generally very satisfied with my Canon 100-400mm Mark II. However, from time to time it will not autofocus when I move from a far subject to a near subject, or the opposite. It's not that it hunts for a while, it simply never gets started. I have to manually change the focus to something close to the subject and then autofocus kicks in. This seems to be happening more recently. Have others had this problem? Is it a characteristic of this lens, or do I need to have it fixed? I had the original 100-400 for many years and didn't have this issue. My camera is a Canon 7D.

Thanks

SteveHS wrote in post #17743023
I am generally very satisfied with my Canon 100-400mm Mark II. However, from time to time it will not autofocus when I move from a far subject to a near subject, or the opposite. It's not that it hunts for a while, it simply never gets started. I have to manually change the focus to something close to the subject and then autofocus kicks in. This seems to be happening more recently. Have others had this problem? Is it a characteristic of this lens, or do I need to have it fixed? I had the original 100-400 for many years and didn't have this issue. My camera is a Canon 7D.

Thanks

There is a custom function that tells the camera whether to keep hunting if it finds AF lock "impossible" (or similar wording). See if that is set to "stop".

Mine sometimes won't AF when I attach it to my 1D MKIV and have it set at 400mm. Like yours it does not hunt, it just doesn't do anything. As soon as I turn it to a lower focal lenght, AF kicks in immediately and for the rest of the day it'll work fine at all focal lengths. It's the weirdest thing!

manual focus to get in the neighborhood. at least that's what I do.

dodgyexposure wrote in post #17743029
There is a custom function that tells the camera whether to keep hunting if it finds AF lock "impossible" (or similar wording). See if that is set to "stop".

I will go with this as the explanation. When you have a very great disparity in where the focus is, and where it should be for the subject, then with longer focal lengths the image is so out of focus there is practically zero contrast to focus on. The reason you might want to stop the lens from driving in this situation, hence the custom camera function, is that if you are tracking a small subject against a plain background, say a bird or aeroplane against the sky, if the AF points temporarily lose the subject you don't want the lens to start hunting by moving from approx infinity to MFD, and back. Infinity to MFD and back can take quite a long time, and while it is happening trying to track the subject can be really difficult.

Alan