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Thanks Robin. Please see my reply to Marcus Ian

I don't think I'm explaining myself too well here. At f/16 the photo IS in sharp focus when I use a target 20m away but the focus is about 2 cm beyond the point where I am focusing. So for instance, if I was focusing on your nose, your eye might be sharply focused but your nose wouldn't be quite in focus. I appreciate that there is more diffraction at f/16 If I did the same test at f/8 and focused on your nose, your nose WILL be in focus as expected and your eyes would be slightly softer as expected.

If it's not an issue how come it is apparent in shots taken at f/16? I first suspected a problem after a safari in Meru last year - I was photographing small birds and almost all of the shots were not quite as sharp as they could be. This was annoying as the AF was focusing right on the eye. I was prepared to accept that some wouldn't be perfect, due to slight movement of the birds and vibration caused by me not having mirror lock up and not having the camera in a tripod, by expected at least 50% would be good. It was nearer 20%. On larger mammals it's not so obvious anyway. The test was run to eliminate the possibility of a lens focus problem, and indeed it has convinced me that at f/8 things are fine. However it was obvious that the sharpest point of focus in the photo did not correlate with where the camera thought it was when focusing at f/16. It's not often I use f/16 in wildlife photography anyway, but I was curious to know if there is a way to fix it. I won't lose any sleep over it, and will certainly be out there taking shots whenever I can.

When checking this lens at 400mm it appears to be spot on at f/8 but back focusing at f/16. Camera is an EOS 7d mk II Is there any way of telling the camera this. I know you can microadjust on camera for different focal lengths but that's NOT what I'm asking.