Calibrating Lens Cams, part I

Calibrating Technika Lens Cams, part I

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.

Linhof Technika IV with 90mm f8 Super-Angulon

How does Rangefinder focusing work on a Linhof Technika?

When a lens is mounted on a Technika and it sits at its correct infinity stop on the rail, and the rail is rolled back as far as it goes, any objects at infinity should be perfectly in focus on the ground glass. Achieving focus on objects closer to us requires moving the lens away from the camera body. In the Technika camera this movement is translated via a number of mechanical linkages to the mirror system of the rangefinder. Because the correct position of the bed for any given focus point varies from lens to lens a unique cam is used to ensure accurate calibration for every lens. The diagram below shows the translation of the movement to the rangefinder. Take note of the direction of each movement from infinity to close focus. You may want to refer back to this to resolve any confusion later on.

The bed moves (1) the cam (2) which moves the arm (3) which shifts the reflected image (4).

List of things to check before starting:

1 Is my camera working correctly?

Having your camera functioning well is the essential starting point. If your camera has an existing lens and cam that is original, you will want to ensure that these are still accurate. If they are, your new lens and cam will be able to be used interchangeably.

If this is camera is new to you or hasn’t been used in a while, it would by good to assess the general condition by following the advice from www.laflexcamera.com. Here are a few more things to look for.

Is my ground glass in the same plane as the film?

Of course we assume this to be accurate, but Linhof have changed the design of the ground glass frame over the years, and things may not be original or not installed correctly. A crude check can be done with caliper, comparing the distance from the front of the frame to the glass to the equivalent distance on a film holder with a sheet of film in it. Another way is to mount a known to be good Rollex roll film back, remove the film holder and ground glass across the film guides. Compare the same focus points with this set up to the 4×5 ground glass and ensure they are identical.

Does the front bed sit accurately at 90 degrees in the basic open position? 

Hold a triangle to the body on left and right side, and verify the angle of the bed.

If the angle is not accurate, that will need corrected.

The arms that hold the bed catches on two concentric studs inside the body. These studs can be rotated to adjust the resting angle of the bed. Lift part of the leather/leatherette hiding the set screws.Loosen the screws behind the concentric studs. Adjust the stud rotation until the bed is level. Match left and right, and tighten the set screws.

Check if the bed sits at 90 degrees on both sides.
The large slotted screw allows rotating of the part that engages the bed arm, which in turn adjusts the angle of the bed. If you need to adjust this take care that you are adjusting both left and right equally. Test it by engaging and disengaging the arms. don’t forget glue the shim cover over the screw before reattaching the leather/leatherette.

2 I have an new lens, a board and a cam. What now?

Technika cams are matched to a lens and will be stamped with the serial number.
Technika IV cams are matched to a body with the serial number stamped in.

Is my lens on the right board? 

Some wide angle lenses were installed on recessed boards. Most lenses will also work fine on a flat board. Note that typically the lens hole is not centered on the board. The hole is positioned lower on the board, to compensate for the fact that the boards sit higher in the body. Boards for wide angles sit higher if they are assumed to be used with a dropped bed. Number 3 size shutters tend to be centered on the light trap ring at the back of the board.

Ensure lens sits parallel, if there is a screw stopping rotation, it should sit in a hole. Note if there are shims.

Unscrew lens ring, and tighten with lens in right position.

Are my infinity stops both at correct infinity position?

Check focus on ground glass in all corners and center. Use a suitably distant subject (100m minimum)

Stops may not be positioned right, and not be equal left and right.

Undo set screw on stop, reposition and tighten. It may be hard to make small adjustments, to the set screw. If Stops are parallel, and close, shimming the lens may be easier.

Loosen the large screws, and adjust the height with the 4 small screws, and lock the position.

Are all my lenses equally accurate at infinity?

Does my existing cam show the rangefinder image correctly at infinity?

You want your adjustments of the camera to result in your existing cam to be perfect. Other cams should be cut according to a set up that is working for your cameras existing lens and cam.

Does my new cam fit well?

Does it slide comfortably, and fit tightly, but also remove easily? Note any imperfections on the casting.

Fit the new cam and ensure it slides all the way to the end smoothly. It should not be too tight or too loose.

Use a blade, or file to remove imperfections that are preventing a perfect fit. Flatten any warping with a flat nose pliers wrapped in tape. Add a slight warp to tighten fit.