Repair: Infinity Calibration Tools (pt1)

Hello, everybody. I will show you an easy way to calibrate your infinity focus here in this post. This technique is for calibrating via the film aperture/film plane and is mostly used for rangefinder cameras and fixed-lens cameras. The SLR lenses and cameras can also be calibrated this way but it’s easier to do it using a Nikon DSLR, read more about it here in my post about how to calibrate a lens for infinity focusing. This is just one of the ways for you to determine whether your lens or camera body is calibrated properly or not and I’m going to show you more ways in the future.

IMG_8212This method involves using focusing screens with a split-prism. A course-ground matte is OK, too but I prefer the convenience of a split-prism. The trick is to rest your screen with the prism side facing the aperture. Mount your lens on the camera and then focus your lens to infinity. Focus on something REALLY far (the moon is also a good idea) and then check the split-prism using a jeweler’s magnifier loupe. Adjust your lens until the subject is in-focus in the screen. This is how I check my rangefinder lenses because my F-mount lenses are easier to calibrate using the way I showed in this link. Just remember to fix the camera in a tripod and then use your hand to hold the screen flat against the film plane of your camera (the shiny rails). You can tape it if you want but I just use my fingers. You want the screen to be flat against the inner rails which is closest to the actual film plane. 

IMG_7637To make things easier, you can glue your screen to the inner surface of a loupe. Be sure that the screen and the edges of the loupe are on the same plane when you glue it or else it’s going to be inaccurate so it’s pointless. If your loupe has an adjustable eyepiece it’s an easy job since you just attach the screen’s housing to the end of the loupe with tape. Just adjust the eyepiece so that you can focus on the screen properly.

IMG_7689Back.JPGImportant: Make sure that you are using the correct side of the screen! The matte/prism side should be facing towards the lens! Also, only place the screen on the inner rails. The outer rails are taller and are meant to guide the film, the inner ones are what’s holding the film flat to the pressure plate. The actual film plane is actually a sliver of a mm lower than the inner rails.

Here are the scenarios for using this technique:

  1. For checking if the flange distance of your camera is correct, mount a lens that you know to be accurately calibrated and then add or subtract shims from the bayonet mount. The shims are usually located under the bayonet mount. Checking this can be tricky because you will have to check the corners of the frame as well instead of just the center in the case of lenses. Shims can be bought from model railroad shops and they come in photo-etched brass with varying thinness.
  2. For checking lenses, just mount the lens on a camera that’s known to have accurate flange distance calibration and then just adjust your lens until the center is sharp at infinity. For lenses without any adjusters, just add shims like I did with this lens and see if that works. If the shim is too thick (which I doubt), just file it down. I wouldn’t file it to be honest because these shims are unique and irreplaceable. The shims are added at the factory and we always assume that the lens is perfectly-calibrated once we put it back the right way. If it’s off then check your reassembly and see if you put it back the right way or not. These lenses are usually made by hand using the adjust and fit method by a skilled machinist using the correct tools and under a very strict quality control system so the tolerances are very small. If this happened to you, it is better to see if you have seated the elements properly than filing something that is irreplaceable. In most cases, the rear element is the one in-charge of focusing so it’s important to check if it’s too far in or out in the objective. To learn about this more, read this article on my Mircro-Nikkor and observe the principle behind it.

In the next part, I will show you another method that I rarely use because I do not have a lot of space in my workshop. I use the method outlined in this article more because of its convenience and I have a window with a view at my workshop. This is also the cheapest way to do this and all you need is a split-prism screen. It doesn’t have to be clean so junks are OK so long as you know that the screen isn’t warped. I recommend using old Nikon F screens because they are cheap. I am not aware of the ones from other manufacturers so you can check those out, too. I am sure that they are just as good and may even be cheap compared to the Nikon ones. The most important thing is that the resulting image should be rendered on the bottom part of the screen. The screens for the Nikon F and Nikon F2 does that. I tried a Nikkormat screen and it didn’t give me the same result.

If you want to follow the method with the loupe, make sure that the it has a rectangular hood and your screens can fit inside. If it’s too big, you will have to add extra materials like acrylics to bridge the gap. I will leave this to you and your creativity, I am sure that you can figure out something smart and if you did, please share it here so that we can all see what you did!

Thanks for reading this article. Did you enjoy this? I’m not sure if this information can be found elsewhere online (English) but this trick can be found in some Japanese websites. I got this trick from an old Japanese camera repair book that I have and I am sharing this with you. See you guys next time and do share this if you enjoyed this. Thanks again, Ric.

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Maintaining this blog requires money to operate. If you think that this site has helped you or you want to show your support by helping with the upkeep of this site, you can simple make a small donation to my account ( Money is not my prime motivation for this blog and I believe that I have enough to run this but you can help me make this site (and the companion facebook page) grow.


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Helping support this site will ensure that this will be kept going as long as I have the time and energy for this. I would appreciate it if you just leave out your name or details like your country and other information so that the donations will totally be anonymous it is at all possible. This is a labor of love and I intend to keep it that way for as long as I can. Ric.


3 Comments (+add yours?)

  1. Trackback: Repair: Infinity Focus Calibration | Richard Haw's Nikon Maintenance Site
  2. Scott Becker
    Mar 19, 2018 @ 02:43:03

    Should note that the 2 pairs of rails are at different, albeit minute, distances from the lens. The film only glides along the inner two rails, the outer two rest directly against the pressure plate. The height difference is approximately the thickness of the film. This ensures that the film gets some relief from the full sprung pressure of the pressure plate.


    • richardhaw
      Mar 19, 2018 @ 07:05:34

      Hi, Scott!
      Thanks, I indicated that it should not be on the film guide rail in the article. That picture confused a lot of people, my fault as I was trying to avoid the screen from falling into the aperture. I will update thay asap. Ric.


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