D800 left focus issue & AF Fine tune - Please help me understand

Started Jun 25, 2012 | Discussions thread
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andreben
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D800 left focus issue & AF Fine tune - Please help me understand
Jun 25, 2012

Hi all,

I know many oF you are tired of all the left AF point issue threads, so I apologize in advance Also sorry for the length post, but please do read on If you think that the issue is user error or non-existent, just move on..

My question to all of you is of a slightly different nature, and I'm mainly trying to understand how the whole AF issue and AF fine thing works. When I first tested my D800 I found that pics taken using the leftmost AF sensor where noticably of of focus, so I decided to dwell more deeply into the issue. I read all (or most) of the threads here, and even though I'm no pro I think that my informal tests at least should be able to show focus inconsistencies.

What I did was to place my camera on a tripod at about 1.5m away from a table. On top of the table I placed a box with a barcode on itit. Immediately next to the box I placed a tilted book to see if focus was dead-on or front/back, the book being just 2cm to the side of the barcode. The barcode filled the entire AF point indicator in the viewfinder (and then some). I manually unfocused to infinity, switched to AF and focus and exposed. I used Mup and 3sec delay, Aperture priority of 1.4. Shutter speeds exceeded 1/500. The lens used for the observations below were the 50mm 1.4D. I also did some simple tripod mounted exposers without Mup and delay, it was identical.

What I found was that to my surprise both the leftmost and the rightmost focus points resulted in significant back-focus while the middle point back-focussed only slightly. So, here come's my first question to all of you:

1: I would assume that the sensor, mirror and AF sensor are all perfectly planar. If so, it's hard to conceive that both left and right back-focuses if it's an alignment issue. One would think that alignment error shifts the left and right in opposite directions. Does anyone understand what the AF issues are all about? Is it a calculation error causing the wrong lens shift based on AF sensor info? Perhaps a manufacturing error with alignment problems (but how is that possible)? Or is it just the nature of the outermost AF sensors and their (in)accuracy?

I also found that the middle point, although quite good, also back-focused slightly (not as bad as left and right). So, I decided to try some AF fine tune. I appled -10 and re-did my test with center, left and right sensors. Again, much to my surprise, all sensor yielded much much better images (center only marginally better and definitely not worse). Baffled, here comes question number 2:

2: How does the AF fine tune work? Does it apply some factor in the lens shift algorithm? If so, can anyone explain how it is possible that the same amount of AF fine tune would improve back-focus on left and right more than in the center, i.e. apparently shift focus further forward on the sides than in the center? Is this just due to the nature of the curved focus field or is there some magic in the algorithms that automatically does this because Nikon knows that focus shift is greater further away from center?

Again, I hope that someone with more knowledge about the AF mechanism and fine tuning could help me understand how AF fine tune can apparently help with my "left/right AF issue" without degrading the center AF! Perhaps I'm not seeing the same left AF issue as others, but I'm still baffled by being able to improve the situation dramatically by performing a AF fine tune of my lens. now I'll try my other lenses

Cheers,
Andreas

Nikon D800
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