Nikkormat Focus Screen Inaccurate

Discussion in 'Nikon' started by henry_finley|1, Aug 22, 2015.

  1. I've got a Nikkormat FT2 here that had a bad screen, so I swapped an FTN screen from a junk body into it. But the focus is off compared to all my other Nikons and Nikkormats. I don't know which direction off hand, but it's about 1/8 inch off in it's turn up near infinity. I don't recall the screen being spring-loaded requiring collimation. I believe I remember the screen dropping in onto a hard stop, like the F and F2's, and not a spring loaded affair requiring calibration.
    So, why is the focus off? Thank you.
     
  2. None of the Nikkormats have interchangeable (meaning they're not intended to be user-changeable) screens. So how did you install the new screen? Unlike purpose-built interchangeable screens, non-interchangeable screens tend not to have any obvious orientation marks - are you sure you installed the new screen the right way up?
     
  3. SCL

    SCL

    It sounds like perhaps a shim is called for, or at least checking to see if the replacement screen is the exact came depth as the replacment.
     
  4. Thank you gentlemen. I am adept at camera repair. The screen I removed was identical in configuration to the replacement, other than the original FT2 screen was a K, and the FTN screen was a microprism. No mention was made in the service manual about a shim, and I've never seen a shim in a Nikkormat.
     
  5. SCL

    SCL

    I once had a similar issue and unscrewed the original screen from its metal mount replacing the new screen in the old mount....everything worked fine...it turned out the metal mounts were slightly different in depth.
     
  6. Hey thanks, Steven Lewis. That's probably it exactly. Unfortunately the old screen appears to not be with me any more to steal the frame off of. And I'm not going to put another nickel into this body shopping around for one. At least I know what the problem was.
     
  7. All focusing systems with a mirror needs calibration. There are tolerances to everything and it's impossible to make two identical cameras. Major adjustment to the distance between focusing screen and mount are made with shims but small adjustments are made with the mirror.
    The register distance between mount and film also have tolerances. One way to compare two cameras is to shoot with a known lens, hard set to infinity, on a tripod. And then mount the same lens without changing anything, on the other camera. Both images should look the same. If one is sharper than the other the register distance is off on one of them. Wide angle lenses shot at large apertures have smaller depth of focus and will show a larger difference between two bodies if the register distance is off.
     
  8. One thing I know from my pursuits in Hasselblad repair, is that the mirror is a non-negotiable 45 degree. From there the screen is necessarily adjusted. Absolutely no mention is made of either adjustment or shimming in the Nikkormat service literature, and I have never seen any in any Nikkormat I ever opened.
     
  9. It's item 315 in the FTN repair manual. It's called Mirror angle adjusting pin.
    45 degree is only a nominal value. The adjustments needed are very small. Image in viewfinder is not visibly affected.
    All modern Nikons have this as well. It's mentioned in several of Nikon's repair manuals that I've seen even if it's not in the FTN manual I have.
    Shimming is the coarse adjustment and mirror adjustment is the fine tuning. Remember that the register distance should be 46.50 mm +/- 0.02mm. That's +/- one thousands of an inch. The distance from the mount to the focusing screen must be the same. So a very small adjustment of the mirror angle will change the distance from the mount to the focusing screen enough to make a difference.
     
  10. Thank you. I am not familiar with you, Pete S, but you sound like you are knowledgeable about this kind of camera work. If you are still reading, is that 46.5mm reading from the face of the lens mount to the film plane? That is, if I put a piece of glass on the film rails and ran my depth gauge down from the face of the lens mount, is this the 46.5 measurement? Thank you.
     
  11. As David said, none of the Nikkormats had user chageable screens like the F through the F4. The position of the screen you replaced may need adjusted with a shim, but just how thick a shim is anybody's guess.
     
  12. If you are still reading, is that 46.5mm reading from the face of the lens mount to the film plane? That is, if I put a piece of glass on the film rails and ran my depth gauge down from the face of the lens mount, is this the 46.5 measurement? Thank you.​
    Hi Henry,
    I'm no expert but I have had problems related to both focus screen calibration as well as problems with the mount and with digital sensor alignment so I'm pretty familiar with these things. The cameras I worked on are newer than the Nikkormats but the principles are the the same.
    I've also discussed focus screen adjustments with Nikon repair and I've watched them adjust a couple of my cameras. Focusing screen is adjusted at infinity with a reference 50mm f1.4 lens and a collimator. Minor adjustments are done with the mirror. If it's far off then a shim is used but that's usually done at assembly.
    Repair manuals for newer cameras also have more measurements and tolerances than the older ones. For instance the mirror is suppose to be 45 degrees nominal but the tolerance are +/- 10 minutes (depending on camera). So everything between 44.83 to 45.17 degrees would be within tolerance.
    The 46.50mm measurement is from the mount to the film emulsion. In the repair manuals it is often mentioned as bayonet to outer rail and then it's another measurement. The only measurement I've seen on this is 46.67 mm +/- 0.02 or 0.03. Of course it depends on the thickness of the rails but it seems like Nikon have made a bunch (or all) the same.
    Some images here and some measurements for other mounts as well:
    https://en.wikipedia.org/wiki/Flange_focal_distance
    Since we are talking small measurements you would need a micrometer depth gage or a dial indicator to get enough accuracy (hundreds of a millimeter or thousands of an inch).
    You might want to have a look at a some repair manuals available on the net. Some manuals have a lot more details than others. You find some here for instance http://www.ss-it.de/downloads.php
     
  13. Well, as it turns out, I lucked up on an FT2 parts camera and robbed the screen out of it Now I have the correct screen for my FT2, and it was a lot closer in calibration than the FTN screen I had in it. After I tweeked that mirror adjustment screw about a half a hair, everything turned out dandy. Thanks, folks.
     

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