Just received this lens a few days ago and had a chance to play with it today. When I use autofocus using my selected focus point (center) and recompose ...once in awhile the camera refocuses. I have it set to AF-S single focus and am using the shutter release button making sure I am not hitting any other buttons..any thoughts. Have used to other lenses on my D800 and have no problems with this. it seems the times it would do it was when I was taking really close up shots and moving the camera just a bit.
Not knowing what the minimum focus distance is of the 85mm f/1.8, is it possible that your initial attempt at focus was on a subject that was too close for the lens, so that no focus lock was achieved; then when you recomposed the camera found something to focus on that was within the abilities of the lens?
---------+---------+---------+---------+ Joseph K Seattle, WA, USA
I had a photo shoot today and I used my 85 1.8G with my D80 and I had the same problems several times. I ended up switching to manual focus. From my understandng, you have to adjust the focusing with the D800 in the camera's menu somehow. Charlie
I agree that the closeness of the subject may be part of the problem. The 85mm f/1.8G has a minimum focus distance of 2.62ft (31in)/ 0.8m.
It is also possible that you are moving your finger just enough when you recompose that you reactivate AF. Try Tom's suggestion to switch to using the rear AF button and decouple AF from the shutter release, at least to confirm where the issue may be. If you frequently select focus and recompose, it is a better way to AF, anyway. You can also try locking focus with the AE/AF-L button.
Thank you for the suggestions. Perhaps it is the distance....even if that is it it seems odd that it would refocus once the shutter is pressed halfway..I made sure it wasn't my finger moving. I have found it seems to only happen when I am doing close-up subjects. I am also finding that once in awhile (even when I am 4+ feet away) I have trouble autofocusing on a subject. It is generally in lower light ..but still plenty for focus with my other lenses. I just want to make sure I shouldn't send the lens back before the 30 day return policy ends.