Sun 13-Jan-13 09:04 PM | edited Sun 13-Jan-13 09:12 PM by ericbowles
The 300 f/2.8 is a very sharp lens. AF Fine tuning may not be required on the bare lens, but still could be needed on one or more teleconverter - or vice versa. It is worth testing under controlled conditions. Every lens and lens/teleconverter combination is independent.
This image appears to be a distant subject highly cropped with branches in front of the subject. My experience is that it's tough to even see small branches to grass that may cause focus in front of the subject. In any event, I would not draw any conclusions about AF tuning from this image and would look at a larger sample of clean subjects. If you have a pattern of consistent front or back focus, fine tuning could be useful.