Auto Focus - Fine Tune

In my quest for image quality I have in the past performed a "Fine Tune" to the AF on my Nikon D3. A few Canons and Nikons, maybe a Sony as well have this feature and it is well worth the time to do the set up. In the past I had used a home made target and was never that happy about the alignment of the target and the sensor. Recently however I acquired a Lens Align Mk 2 target and decided to go through all the AF lenses again to check their performance at auto focusing. It is disappointing that nearly all AF lenses do not do focusing that well. Some are spot on but some will back or front focus by as much as 20mm in my experience. This on many occasions will never be noticed ( wide angle lenses and when using say f8 due to the improved depth of field) but when working close to minimum focusing distance and wide open with a fast lens the lack of sharpness where the focus point should be can be puzzling.

The front view of the target has focus area and alongside it a sloping ruler to determine where the actual focus lies. The centre AF point is aimed at the main target for testing, with the rear alignment just visible through the small hole in the centre.

In the photograph below (600mm f4 at f4 at 25 times focal length distance) the focus is clearly behind the target. Adjustment is made in the camera menu and the camera lens combination can be forced to focus correctly. The camera memorises the serial number of the lens and changes the adjustment to suit each individual lens and up to 20 can be stored. Images are made in monochrome to allow ease of assessment and against a dark background and maybe over sharpened to evaluate the focus area. Trial and error is required and the lens should be sent through it full movement range between each test.



A chore but the results are noticeable when completed.