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10-06-2014, 10:34 AM   #1
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Focusing Screen shims?


I bought a focus screen from It vas a K3 NIKON FM3A screen they had cut to fit my K-x camera.

Following the instruction for mounting was somewhat confusing. The instruction showed how to first take out the shim, and then to replace it with a simular looking yellow metal shim.

I had recieved two half transparent plastic shims, and that made me uncertain. I tried with both one and two shims, but with two shims mounted one of them tended to slide out of place.
I did not like that, so I tried with the original shim. Then I measured focus with macrophoto of a ruler and after a fence at a few meters distance. Seemed perfect.

But, even if focus comes perfect at short range, it seems like there could be a fault that increases with distance. That might be caused by a faulty mounted focusing screen, I suppose.

What is the correct shim to use? One or two plastics, or the original yellow metal shim?

10-06-2014, 11:07 AM   #2
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It's been awhile since I put a screen in but the answer would be whatever focusing screen tells you. All screens will be of slightly different thickness, especially when using a screen not made for Pentax. So they include shims to make it work. If you tested it and it works I would say you are good. I spent a lot of time on mine (years ago) and made my own shims from pieces of post it notes. I would hope that they would be better at supplying the correct shims after doing this for so many years.

If the original works then I would just go with that. Not sure what you mean by a 'fault that increases with distance'; either the screen is shimmed right or it isn't, it will not vary with distance.
10-06-2014, 11:26 AM   #3
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If the shim you used provides correct manual focus at closest distance possible (and the lens wide open) it will properly manual focus at all distances (and wide open). Some lenses [especially very fast ones] have a focus shift as you close the lens aperture down--but that is another matter and is not (normally) part of the shim adjustment process.

---------- Post added 10-06-14 at 11:31 AM ----------

BTW I purchased one of their screens [for a K-20d] and found the plastic shims to be problematic (the screen itself is excellent), and in the end ordered a range of sizes of metal shims from Pentax USA. Unfortunately (I gather from recent posts) Pentax/Ricoh no longer sells them.

Last edited by dms; 10-06-2014 at 11:31 AM.
10-06-2014, 03:50 PM   #4
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Have you got the shim(s) in properly under the shim holding catch ?

10-06-2014, 04:03 PM   #5
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Question #1...Did you check to see if you need to change the shims?

If yes,
Question #2...Were you able to determine if you have front or back focus?

Instructions here: Focusing Screen--How to adjust focusing screen--

If back focus, you add additional thickness to the stock shim (if any) between the screen and the pentaprism.

If front focus, you remove the existing shim and replace it with thinner stock.

On the K-r, there is no additional retaining clip for the shim.

Edit: Forgive me for not reading your original inquiry carefully. If the stock shim yields accurate focus, simply put it back in and be happy. If not, reference the struck-out text above. I suggest that you test it (camera on tripod) against a nice, high contrast target with a strong vertical line. Test at about 20x the lens focal length and use a lens originally designed for manual focus, if you have one. A longer focus throw is easier to work with.


---------- Post added 10-06-14 at 04:09 PM ----------

You may also want to review the instructions on the KatzEye site. They have excellent photos and also include detailed information regarding the shim.

You should not have to remove the stock shim (if any) unless there is a front focus problem. I don't know why includes that step, since they have you put it right back in. If you need additional plastic shims, contact Pentax no longer sells these parts directly to consumers.


(...apologies to Rachael Katz...)

Last edited by stevebrot; 10-06-2014 at 04:25 PM.
10-07-2014, 12:39 AM   #6
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Thank you for ensuring me that I got it all right anyhow!

It is an assumption of mine, that a small early error will be enlarged with focal length and distance.
The reason I asked you, is that I have had a problem focusing with a Kennex 400/6.3 (supposedely a Soligor)

with a T2 mount. And I find it lousy at distances, focus ending up behind what's intended. So, was that due

to the lens or the shim?

Now I can know for certain. Thank's a lot!

And Steve
Thank you for the link to /test-fsen, I missed that one, it explained everything!
10-07-2014, 05:43 AM   #7
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QuoteOriginally posted by Bromid Quote
So, was that due

to the lens or the shim?
It may be the shim. For manual focus, there is no need to do lens-specific lens calibration. Do you have a fast 50mm manual focus lens? That would be preferable for assessing your focus accuracy. Do your testing at 20x the lens focal length using a flat target with a nice black vertical line. Assess the focus using magnified live view (if your camera has it) or an actual image. Do not test against the AF focus confirm feature.

12-04-2014, 07:08 AM   #8
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Finally done.
When back-focusing with a 400 mm M42lens (600 mm digitally), I inserted an extra plastic shim, but instead got it front-focusing.
With 50 mm at f:1,8, a ruler at short distance turned out perfectly.

Cut a thinner shim of aluminum foil, that I glued to the original shim, and got it just about as right as I can deem.

Thanks for your supportive suggestions!


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