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Archive 2012 · How to fix back focus for Nikon D90?
  
 
Smridevan
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p.1 #1 · p.1 #1 · How to fix back focus for Nikon D90?


I recently bought a Katzeye split prism focus screen for my D90 to use with my MF Zeiss lenses, but I'm having back focus issues. What I see in the viewfinder is in focus but in the actual, captured image the rear is in focus. I purchased the D90 new and I see there are two shims already there when I took out the stock screen. My questions are: 1) do I need to add a thicker shim or take away one of the shims? and 2) if I need a thicker shim, where do I buy this? Thanks!


Feb 18, 2012 at 08:14 PM
theSuede
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p.1 #2 · p.1 #2 · How to fix back focus for Nikon D90?


Add shim thickness to reduce back-focus.

Shims can be ordered from Nikon (if they still accept non-qualified persons / companies to order...) - Try contacting your nearest Nikon service center. They're just a phone-call away. Otherwise try Katz themselves.




Feb 18, 2012 at 08:30 PM
Smridevan
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p.1 #3 · p.1 #3 · How to fix back focus for Nikon D90?


Thanks...I just called Nikon technical support and they had no idea what I was talking about and was told to find it and order it on the website. I didn't see it on the website. I'll double check to see if Katzeye sells shims...


Feb 18, 2012 at 08:53 PM
Smridevan
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p.1 #4 · p.1 #4 · How to fix back focus for Nikon D90?


Just to update on this issue for those who might have a similar issue.

So after e-mailing Katzeye, they quickly responded and provided details on how the adjust the viewfinder using the manual focus adjustment screw behind the mirror in the camera. The instructions were similar to the instructions on this website:

http://www.howtoshot.com/82/how-to-fix-front-focus-nikon-d90-d80-back-focus-problem/

warning: just don't touch the AF screw towards to back near the sensor! Only adjust the MF screw close to the lens mount.

The adjustment works beautifully and the AF is not affected at all. This is awesome!






35mm 1.4 before adjustment

  NIKON D90    35mm    f/1.4    1/400s    200 ISO    0.0 EV  







35mm after adjustment

  NIKON D90    35mm    f/1.4    1/320s    200 ISO    0.0 EV  




Feb 28, 2012 at 04:06 AM
zhangyue
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p.1 #5 · p.1 #5 · How to fix back focus for Nikon D90?


I didn't spend more time to reading it, but I won't do that, unless you always shoot subject in center.

You can't tilt it. you need shimmer to adjust parallel distance.

If you have hard time to find it, just pick a hard piece of plastic sheet, thin enough, and cut it carefully. Your D90 should have shimmer pre_installed if like D300s and D700 I have. Use that as a example.

while, as I am writing, this case may be OK since you use split screen. and most likely your subject is in the middle. That is why I don't like use it, instead of Canon Type-S screen, which is very handy for fast glass and you can put your subject anywhere in the frame.

Still, it will bother me that my focus screen is not parallel with my focus plain.(or i should say equivalent since you didnot tilt scree but mirror) And I am not even sure will adjusting mirror mess up the AF. Hopefully not, as there is separate screw for AF. But who knows? You may verify after it.





Feb 28, 2012 at 06:51 PM
 

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espressogeek
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p.1 #6 · p.1 #6 · How to fix back focus for Nikon D90?


Awesome info. Perhaps I can pick up one of these screens for my D800E when it arrives.


Feb 28, 2012 at 06:59 PM
zhangyue
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p.1 #7 · p.1 #7 · How to fix back focus for Nikon D90?


espressogeek wrote:
Awesome info. Perhaps I can pick up one of these screens for my D800E when it arrives.

That is the thing I am not so sure as from spec, look like D800 has the same screen as D3/3s/4, for some reason, there is no S-type screen will fit. Hopefully, things will change once D800 hit market.



Feb 28, 2012 at 07:12 PM
Smridevan
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p.1 #8 · p.1 #8 · How to fix back focus for Nikon D90?


This adjustment did not require using shims because the back focusing was relatively small. Perhaps the non-planar focus plane issues you have might be a result of a focusing screen that is not perfectly flat due to manufacturing tolerances or that the focusing screen was not seated perfectly flat during the installation. Making your own shims would be very difficult or impractical to get it perfectly flat and even without some high precision machining tools. It might introduce more non-parallel focusing plane issues.

I believe Katzeye's focusing screen has pretty good flatness and their screen is recommended by Zeiss (or at least Zeiss' website provides a link to Katzeye for focusing screens).

Although I haven't tried the Canon type-S screen, I think Katzeye's screen with the split prism center focusing aid works pretty well for focusing in the center and recomposing. For medium to large distances, the focus plane should not change too much when recomposing to affect the focus.



Feb 28, 2012 at 07:25 PM
zhangyue
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p.1 #9 · p.1 #9 · How to fix back focus for Nikon D90?


This has nothing to do with the flatness, and we are not talking the precising mechanic here

There are three paths the light goes into your camera: A: sensor, B: AF module, C:Focus screen to your eye.
Once distance A=B and A=C, you are fine. If not you need adjust.

If A=B mess up, you need micoadjust your body. If A=C mess up, you need adjust your screen.

If your screen tilt or AF module tilt because of manufacture tolerance, you need adjust those screw, but most likely, Nikon has done that before units are for sale. As I said, unless that is verified, I won't do that.

Your D90 screen has different thickness than Katz, thus the problem.

Here you adjust the mirror angle to alter the light path, the consequence is focus plain no longer 90 degree to your focus screen. Lets put in other way, if you have type S screen, only one horizontal line in the middle in focus will insync with your sensor, if your subject is upper or lower, either it will be front focused or back focused if you rely on your screen.

Again, I think you maybe fine in this case, just want make sure it doesn't alter your AF sensor path as I am not quite sure about that.

hope this helps.




Feb 29, 2012 at 01:44 AM





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