Front focus and Back focus manual adjustment


Front or back focusing can be a serious problem. You buy new Nikon camera and suddenly does not focus properly with old lenses. Very frustrating. And even more when you take all to the Nikon service and after one month they will send it back with result “everything is in limit”. So lets talk about D90 back/front focus adjustment. How to adjust autofocus of your camera in 30 minutes.





Nikon DSLR Autofocus and View Finder system


This article is based on several forum posts and especially original article of Sergei Khizhiy about back focus problem on Nikon D70 and later leongoodman.com. Links to the original articles are at the end. Important thing is that this method is not just for really strong back/front focus problem however also for fine tunning the camera. Even if you get the answer that everything is in limit or your camera works fine with expensive ethalon prime lens (which you obviously dont own). This can help you to get better focus with your current lens (if it has some focusing problem). Important note: AT YOUR OWN RISK. THE FOLLOWING PROCEDURE HAS NOT BEEN APPROVED BY NIKON

On the left you can see basic design how the autofocus system of the D90 works. There are two mirrors. One for autofocus (AF) and one for manual focusing – viewfinder focus (VF). The general idea is that screw 1 and 2 are easy possible to adjust by hex key screw driver (also called Inbus or Imbus). And because they are even not hidden or covered, obviously they are here for adjusting purposes of the service.

When you frame a picture, the light from the lens travels along path A (green). A fraction of the light beam goes through the semitransparent rectangular window in the main mirror and is reflected to the auto focus detectors (path C, magenta). When you take a shot, both VF and AF mirrors lock at the upper position, and the light hits the CCD directly by path B (yellow). Distances A, B, and C must be equal to some base distance D defined by Nikon engineers for their SLRs long time ago.

Because its is impossible to fine tune or even change the positions of the CCD/CMOS sensor or AF sensor, we have to find another way. There is also no way how to change this by software (so far from what i know), so this is the only way i find on the net. All you can is to change distances A and C by changing just a little bit the working angles of the corresponding mirrors. Basically, the mirrors should be positioned precisely at 45 degrees from the main optical axis of the camera and lens. These angles are maintained by the positions of the eccentrical stoppers, fine tuned by hex key screw 1 and 2. you can easily find them on the right side of the mirror chamber. This general design seems to be the same for Nikon D70 D80 D90. I do not know if its also for D7000.

1. Print a good test pattern like the one used for testing at www.dpreview.com (ISO 12233). Stick it to a flat vertical surface in front of the camera and light it brightly to help manual focusing.
2. Remove the lens, pull up the mirror, lock it in the upper position with an adhesive tape for convenience, mark the initial positions of the stoppers for a roll back, take a 2 mm hexagonal L-shaped wrench and rotate the AF stopper against its axis to a chosen direction by a few degrees. Then unlock the mirror and put the lens back.
3. Mount the camera on a tripod and align it carefully against the test pattern.
4. Set the Aperture Priority mode on the camera and use the fastest aperture available.
5. Let the auto focus do its job and shoot. Then shoot two more pictures of the pattern with a manual correction of the focus by half a notch to either direction of the focusing ring (focus bracketing). The kit lens of the D70 allows you to do so simply by turning the focus ring while pressing the shutter release button half way. Check the user manual on that.
6. Download the images to a computer and see which of the images is better: autofocused or manually corrected. This will give you a clue if you need to rotate the AF stopper some more or if you rotated it to the wrong direction and need to reverse it. Repeat steps 2…6 until you get a perfectly autofocused picture.
7. Now that you have done with the auto focus, proceed to the calibration of the manual focus. It is done by turning the VF stopper to a certain direction by a few degrees. Keep adjusting the VF stopper position until you get the two following things at the same time: first, you must see the test pattern through the viewfinder perfectly in focus, and, second, you must see the confirmation sign of the auto focus (a green dot lit) in the bottom left corner of the viewfinder.
8. Check the result by downloading images to a computer. Use focus bracketing to see that you get the best resolution with auto and manual focusing and without focus correction.

Here is the original Sergeys procedure:

1. Print a good test pattern like the one used for testing at
www.dpreview.com (ISO 12233), for more see links at the end of this article. Stick it to a flat vertical surface in
front of the camera and light it brightly to help manual focusing.
2. Remove the lens, pull up the mirror, lock it in the upper position
with an adhesive tape for convenience, mark the initial positions of
the stoppers for a roll back, take a 2 mm hexagonal L-shaped wrench and
rotate the AF stopper against its axis to a chosen direction by a few
degrees. Then unlock the mirror and put the lens back.
3. Mount the camera on a tripod and align it carefully against the test
pattern.
4. Set the Aperture Priority mode on the camera and use the fastest
aperture available.
5. Let the auto focus do its job and shoot. Then shoot two more
pictures of the pattern with a manual correction of the focus by half a
notch to either direction of the focusing ring (focus bracketing). The
kit lens of the D70 alows you to do so simply by turning the focus ring
while pressing the shutter release button half way. Check the user
manual on that.
6. Download the images to a computer and see which of the images is
better: autofocused or manually corrected. This will give you a clue if
you need to rotate the AF stopper some more or if you rotated it to the
wrong direction and need to reverse it. Repeat steps 2…6 until you
get a perfectly autofocused picture.
7. Now that you have done with the auto focus, proceed to the
calibration of the manual focus. It is done by turning the VF stopper
to a certain direction by a few degrees. Keep adjusting the VF stopper
position until you get the two following things at the same time:
first, you must see the test pattern through the viewfinder perfectly
in focus, and, second, you must see the confirmation sign of the auto
focus (a green dot lit) in the bottom left corner of the viewfinder.
8. Check the result by downloading images to a computer. Use focus
bracketing to see that you get the best resolution with auto and manual
focusing and without focus correction.

Some links to useful articles I get most of the information: Here are front/back focus test charts. There are several ways how to do it. Some people like the lines, another tiny texts. My suggestion is that you need both.

Note: I post this article because I have difficulties to find relevant info about front focus Nikon D90.

Note: I post this article because I have dificulties to find relevant info about front focus Nikon D90.

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