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never ending focus adjustment thread

nikonlinks

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nikonlinks Basic Member
Thu 25-Jan-07 01:52 AM

This post is in followup to many previous, both mine and others, about adjusting the body for focus accuracy, using the mirror stop screw. Several members have emailed asking the question "does it work". And the answer is...



Long story: The procedure of adjusting the mirror is WRONG, and is not the right way to fix the problem. Those who recommend against the procedure are right. The Nikon service manual explains the correct adjustment procedure, and you cannot do it yourself.

Short story: The mirror adjustment procedure is so close, that it often works anyway.




Since performing the mirror adjustment, I have since purchased the actual Nikon service manuals for D70 and D2h/D2hs. By the manual:

- The mirror adjustment screw is only used to be sure the mirror is at an exact 45-degree angle. To measure that, you need to disassemble the body, use a special Nikon calibration tool, special Nikon focus screen. That does make sense, because if the mirror were at any angle other than 45 degrees, the focal distance at the top of the screen would be different than the distance at the bottom of the screen. In such a case, the focus accuracy might be perfect on the center horizontal row of points, but would be off using an AF point above or below that line. The same would be true with manual focus.

- To adjust manual focus accuracy, you change shims in the focus screen mount. These would be very small shims indeed, because even 2 thousands of an inch would have a noticeable change in focal distance. You also have to check the manual focus distance at various points on the screen, so the shims are not the same all the way around.

- To adjust the auto focus accuracy, you hook up a computer with special diagnostic software. It is not a mechanical adjustment at all.


Although the procedure I used is definitely wrong, my result was ok because the amount these devices physically move in the adjustment is so tiny, my adjustment of the mirror angle is probably within Nikon's field tolerance. Technically, I introduced a new error, to compensate for a different error. Moving the mirror eccentric the tiny amount I did, my mirror may now be at 45.5 degrees (example only - I can't measure it), and that error is cancelling out the initial setting error from manufacture. Its entirely possible on a particular camera that moving the mirror one direction might help the manual focus, and make the autofocus worse. As you move the mirror, the indicated autofocus points will become inaccurate, although again with the small amount I moved it, I haven't seen that as a problem.

If your camera is under warranty, certainly let Nikon fix the problem the right way. If its not in warranty, you really have nothing to lose by trying, because if you screw it up and later send it in for calibration, they will check all the adjustments anyway, and the price is a flat fee.

There have been several reports of people sending their cameras in more than once for this problem, with little improvement. There's no such thing as perfect, and Nikon's tolerance is sometimes not up to a particular user's demands. In particular, changing the shims for manual focus is not a continuous adjustment (its in steps). How close can they get it with that method? That actually happened to me with the D2h. I sent it to Nikon for the meter repair and focus calibration. The focus was still not to my liking.

I am personally very happy with my results after doing the mirror adjustment myself, on both the D70 and D2h. Any of the possible inaccuracies I noted above, are apparently very small in comparison to the error that I started with. I have used both bodies with a variety of lenses with shallow depth of field, including 50/1.4 and 500/f4+2x, both manual and autofocus, have focused with all the various AF points, and manual focus around the screen. My actual pictures are much sharper, so I'm happy. Your results very well could be different.

don ferrario
www.donferrario.com

don ferrario
www.donferrario.com

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