Calibrating Lens Cams, part I

How to cam a lens for a Linhof Technika.

The following description is a method for filing a un used focusing cam for your lens in order to get accurate rangefinder focusing on a Linhof Technika. This guide uses a 4×5 Technika IV, but the method can be applied to other Technika models, including 6×9 and 5×7 models. This guide is broken into 3 sections covering checks and preparations to your camera, fitting of a new lens and matching cam, and the process of calibrating the cam to get accurate focus.

It is possible to achieve a very high degree of accuracy with sufficient care and diligence. Before starting it is good to prepare by having the right tools, and a good place to work. It is also essential that the camera is working well with the lens you are trying to cam. Material you remove from the cam can’t be replaced, so if you take too much away you won’t be able to go back and redo it. 

Linhof Technika IV with 90mm f8 Super-Angulon

1 Is my camera working correctly? 

Having your camera functioning well is essential for your effort not to be wasted. If your camera has an existing lens and cam, you will want to ensure that this is working well. You don’t want any adjustments you make for one lens to mess up settings for another lens.

If this is camera is new to you, it would by good to assess the general condition by following the advice from The following are a number of things to check.

Is ground glass in the same plane as the film?

Use caliper on film holder and on ground glass.

Earlier Linhofs may not be standardized. I have not encountered this issue, but it doesn’t hurt to check.

If needed you could add shims under the ground glass. I like to use layers of copper tape, that has a peel off sticky side.

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.
Check if it is 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.

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.