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Lens and Optics
Checking Lens Working Distance


You've done some work on a lens, either cleaned it or dismantled it for some reason. Now to finish it all up by making sure it will focus accurately.


Most lenses will have some sort of shimming method to allow you to fine tune the distance the lens sits from the film plane. By either adding to the shim stack or reducing the thickness you can dial-in your lens. Don't accept the factory's haphazard approach to this important detail as they did not take as much care and attention that you would expect.


Use a camera with known accurate lens mounting flange to film plane distance, see the Cleaning & Repairs section Checking Camera Working Distance on how to achieve this. I use a focus screen that has been cut down to size to fit between the outer film rails. Attach the modified focus screen to film rails. Now the focus screen will indicate exactly what the film would see. One of the cool things about the Kiev 4 line of camera is the ability to mount to a tripod with the back removed. Here I have a Manfrotto base attached to my Kiev 4.


You will check the focus at both infinity and at 0.9m distance with a 10x optical loupe. For infinity take the camera with the lens in question outside or if you are fortunate enough to not have too many trees in the way aim through a window at a distant object with distinct vertical lines. I sight on a transmission tower about 1.5km away. The tops of pine trees work too. The focus screen I use has a split image focus spot which greatly aids in checking.


For checking at minimum distance point camera down at about 45 degrees to a table top and lay one of these handy focus test targets about 1m distance from the camera. Big thanks to John Wilton for creating this focus test target. You can find a copy of this test target at: http://www.ragarecords.com/photo/index.html.  Using the camera's rangefinder focus at the center line on the target scale. As an aside, this procedure relies on you having an absolutely accurate rangefinder, this is another procedure in Cleaning & Repairs called Checking Rangefinder Accuracy. If the lens agrees with the rangefinder then the center line will be in focus, if not you can see whether it is too far or too close.


If your lens does not exactly focus then determine whether the lens is too close or too far from the film place and take the appropriate action with the shims. Use the existing shim as a tracing stencil and cut a new one. Sometimes all you need is the thickness of Aluminum Foil (0.02mm) or even paper (0.12mm) to get the proper shim distance. If the existing shim is too thick then gently remove some thickness by sanding in circular movements with 400 grit wet-or-dry sand paper. The existing shim is usually made of stamped aluminum so it is soft and easy to work.

You may have to do a few trial and error goes at it but it will be worth the time spent knowing that your lens is absolutely spot on.

November 11, 2008