S.K. Grimes - Leica Rotating Drum Camera Shutter Tester

A good and simple way to check the operation of focal plane shutters has existed since the earliest versions of these shutters.

This one is the version as used by Leica workers in their factory and service stations.

Consider the operation of an ordinary 35mm focal plane shutter. The two traveling curtains make a slot which travels from right to left as viewed from the back of the camera. This slot is not a fixed width, but depends on the good adjustment of the camera. At 1/1000 second its about a millimeter or so wide and should remain that width across the focal plane.

By arranging an apparatus so that a brightly illuminated slot of fixed width and velocity is presented at right angles to the variable width and changeable velocity of the two camera shutter curtains the problem is solved. All parameters, (velocity, acceleration, slot width and even rebound (bounce))of either curtain can be easily examined as an aid to adjustment. A cylinder, about six inches in diameter with slots 1mm wide about 1/2" apart (evenly spaced) turning at about 280-300 RPM will do this.

This shot shows clearly how simple this is. Although the Leica instrument is extremely well made and uses a very nicely balanced drum driven by a synchronous AC motor, my experience with home-made versions is that none of the dimensions are particularly critical to the operation or accuracy of measurements. The Leica version is reminiscent of a well made record turntable, using a rubber tire capstan drive on an adjustable slope to vary the speed slightly: Belt or direct drives seem to work just as well, in my experience.

I first saw one of these testers as a guest for training at the Leitz repair station in Rockleigh, N.J.. (I went on to be part of the Leica Recommended Repair Station program till it was discontinued) I was immediatly enthralled by the design and persisted to be allowed to partially disassemble one of the testers and make sketches and measurements. I felt as if I was being allowed a look at the workings of the Enigma Cipher machine!! I was able to make and experiment with my own home-made version to fully understand the capacity of this wonderful instrument.

How to Read and Interpret the Tests

The instument is used by placing a camera with no lens and the back open on the shelf at the front. The operator winds and clicks the shutter while looking at the focal plane rectangle. The tester gives useable results at 1/1000 second to 1/125 second. Consider again the expected phenomenon of the two slots moving at right angles to one another. The trace you see at the instant of exposure looks like a series of slightly curved bands of light, each band corresponding to the intersection of one of the slots on the drum with the slot of the shutter. The Leica tester has a green glass window which causes the trace to appear as an electronic/oscilliscope looking artifact.

The picture above is a good representation of what you see on an ordinary Leica type focal plane shutter in good adjusment. The green bands are wider as the slot is wider. At 1/125 second they are so wide that they begin to overlap but are still useable. Use at 1/60 is marginal. And at speeds below 1/60 some other method must be used.

These show how too slow, too fast and unbalanced curtain tensions trace. The upper margin of the green trace corresponds to the opening (first) curtain velocity, The lower margin of the trace indicates the closing (second) curtain velocity. The higher(faster) the velocity of the curtains the more nearly horizontal is the trace. The lower (slower) the velocity the more nearly vertical is the trace.

Other diagnostics follow. Consider always the phenomemnon of the fixed slot turning at constant speed at right angles to the variable slot of the camera shutter under consideration. Breaks, irregularities or too great or too shallow a curve are all information about bearing and brake condition and spring tensioning.

This tester depends on practice and experience to interpret the results. It is, however, highly intuitive, and experimenting with cameras in good condition on a home-made version will quickly render it very useful and highly versatile.
The use of such a tester by manufacturers also explains why, especially on older models no data about curtain travel times in milliseconds is available. Everything is done by "faster" and "slower", "wider" and "narrower" Its fun to make one of these and figure out how it works!
Inspections and adjustments become like playing a violin by ear -- you don't need to know what the frequency of C-sharp is, you just learn to play it in relation to the other notes.

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