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03-18-2012, 09:00 AM   #1
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Pentax MX focusing screen at infinity problems

I recently cleaned the focusing screen of my MX with just a blower. Now, the split prism no longer aligns when focusing to infinity. For the split image to align, I would need to focus past infinity. I tried reinstalling the screen multiple times with no success. This occurs with a variety of different lenses. I looked it up, and apparently I would need to take out shims to fix the focusing problem, but, there are no shims there to begin with. I looked up by the prism and there doesn't seem to be anything blocking the screen from sitting flat up top.

I also read that the mirror could be misaligned? When I gently push on the mirror, there is some give before it hits a stopping point. Could this be it? How durable is the prism itself? Could I have damaged it during installation? Thanks for any tips.

03-18-2012, 10:02 AM   #2
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A colleague told me he had to adjust his camera's mirror position (not a Pentax MX) to get optimal focus but I can't see any adjustments for this in the service manual -> http://www.robertstech.com/manuals.htm
I got the MX manual to try to service the timer once but just never got around to it.
03-18-2012, 12:08 PM   #3
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A focusing screen installed upside down or bad mirror stops could cause viewfinder focus inaccuracy.

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03-18-2012, 02:04 PM   #4
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If the focusing screen is in alignment (yes, I know that's a big if) could it be that the cleaning has just made obvious the fact the photographer's vision needs optical correction?

03-18-2012, 05:20 PM   #5
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QuoteOriginally posted by ChrisPlatt Quote
A focusing screen installed upside down or bad mirror stops could cause viewfinder focus inaccuracy.

Chris
That was my first thought Chris, but was hesitant to suggest it.


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03-18-2012, 06:24 PM   #6
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If you did not remove the screen from the carrier, then did lock properly secured? The blower might have loosened the carrier and it might not have been locked proprely to begin with. You could check the latch and even open the carrier and try to reseat it. The carrier had a little tab that goes through the latch. Also, check with a bright light... first put the body upside-down and look at the screen by shining a flashlight at the mirror and looking at the focus screen's center. It should look smooth and continuous... if it is the split/microprism version. If you side the textures and details of the focussing aid, then it is installed backwards.

Note: In some of the earlier MX bodies, some quality control oddities existed and that included some mirror hinges drilled crooked, also meaning the carriers were a little off as well. My early MX from years ago (rest in peace) had all these issues. My current MX bodies show none of these.
03-18-2012, 06:27 PM   #7
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It's definitely not upside down (only one way to install it with the tab in the right position). How can I fool around with the mirror stop to determine if that's the problem? Thanks Les for the repair manual, seems like I can also change the focusing screen frame itself. It does seem like it's not that tight when clipped in, as there is still some vertical play as I can push the frame up further when it's clicked in. Unfortunately it doesn't give a good illustration on how to take it apart and adjust it.

QuoteOriginally posted by asaru Quote
If the focusing screen is in alignment (yes, I know that's a big if) could it be that the cleaning has just made obvious the fact the photographer's vision needs optical correction?
Bad vision shouldn't affect the split image from lining up correctly if I'm not mistaken, although it is true if I had poor eyesight it would still look blurry to me. But I have contacts, and with them perfect vision so that's ruled out.
03-18-2012, 06:44 PM   #8
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MysteryOnion: looks like quality control is the culprit. The plate that holds the focusing screen from above is crooked. Looking into the MX, the right side drops lower than the left side of the plate. This seems like the plate that is described in the repair manual Les mentioned. The problem is getting to those screws. I previously shot with wide angles and my fisheye on this MX, so I probably just overlooked this focusing screen problem until now, when I tried shooting with my telephoto.

Not sure if I should risk trying to fix myself, or sell on ebay mentioning the problem and then buy another one...

03-19-2012, 03:22 AM   #9
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Usually Eric is the guy to fix it for you, but given your location it may be very expensive for shipping?

The carrier can be pushed home to the latch if you are careful and use a thing edge device on only the tab postion portion. I once had a bent tipped metal tool similar to the tool supplied by Pentax for the screens... just bigger. If the focus error is all, then the proper seating of the carrier may solve it. You should check first if the screen is centered in the carrier as the QC issue causes the back postion to hand a bit. I used to hold the camera upsidedown and lift the carrier to allow the screen to fully rest in the screen space and then position the carrier back to lock it down. The thin tool was made to work only on the latch and tab as to not slip and scratch the screen. A nail file ground flat and an eighth of an inch bent would be a close stand-in.

Last edited by MysteryOnion; 03-19-2012 at 03:30 AM.
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