Calibrating Technika Lens Cams, part II
This is part 1 of a guide in 2 parts on how to calibrate a focussing cam for lens on a Linhof Technika in order to get accurate rangefinder focusing. The images show a 4×5 Technika IV, but the method applies equally to other models including 6×9 and 5×7 cameras. Blank (uncalibrated) cams can be ordered from my Shapeways shop. Used cams found on ebay may also be used with certain limitations. Part 1 is of this guide is a check list to ensure that your camera is function optimally. Any inaccuracies in its general functioning can adversely affect the accuracy of the rangefinder, so let’s start with first things first. We will also ensure the and cam are fitting. Part 2 describes the procedure for filing the cams for your lens. Beware that this method is laborious, and will require a good deal of your patience, but if you work diligently it is possible to achieve a high degree of accuracy.
A soft pencil, a white board marker. Eraser, or tissue for wiping marks.
Flat metal files of various roughness. Fine grit sand paper (600+).
Pliers, masking or artist tape and a tape measure.
you will want to work somewhere that has a good view to infinity. For shorter lenses 100m /300ft may be sufficient, but the farther your infinity is the better. Check on the ground glass with the lens to see if you can distinguish between distant points. If you can, pick a more distant object. You want to be able to set up a tripod and easily angle the camera to include nearer focus points. 3 or 4 points that are evenly spaced between infinity and 1m /3ft will do. For example a sequence like: infinity, 15m (45ft), 7m (20ft), 4m (12ft), 2m (6ft), 1m (3ft.) These distances are approximate, as accurate checking will happen on the ground glass.
You also want enough room to work, with lenses, and tools.
Camming the lens
After the pre-check sequence, you will work from infinity to close focus. Start by approximating the range of focus points, across the range. Then go back over the range to get each position closer to accurate focus. It is important to note that material filed away can’t be replaced. Work carefully and diligently.
Find a place to work that has a good subject at infinity. Preferably several 100m away. The next point can be 25-50m, then 15m, 10m, 5m, 3m, 2m 1m. The exact distances are not critical, just ensure that they are evenly spaced along the curve.
Focus the lens at infinity object on the ground glass. Lock the focus, and if needed slide the lens stand into the body to get access to the cam. Mark where the wheel hits cam.
Find the infinity object with the rangefinder. Mark where the wheel hits the cam.
Compare the two markings. The Rf mark represents the angle the cam needs to be at infinity. The GG mark represents where the wheel needs to be at infinity. Remove cam from camera, and start filing material away in between the two marks.
Wipe the marks. Refit the cam, and repeat the process to make new ground glass and cam marks.
Notice how the marks are now closer together. Repeat the sequence until they are 1mm or less apart.
Once you are close to infinity, find your next nearer object to focus on and repeat the marking sequence.
Use a flat file and position it between the marks, and remove some metal. Repeat the checking procedure until these are close together.
Work your way past all your focus points down to your near focus mark. You now have the approximate shape of the curve. Always be wary of removing too much material.
Starting at the infinity mark cycle through all your focus objects to see how far off they are from accurate focus. Mark where each focus point falls on the cam, making a heavier mark where it is less accurate.
Working with a finer file, and finally with fine grit paper file down gently and check frequently until each point is accurate. You can now also focus on objects at intermediate distances, to verify that your points were accurately bridged. Making the surface smooth with a fine grit paper ensures that the cam will glide nicely.