The Leica Rangefinder and the importance of a well calibrated system – By Alan Starkie

The idea of the rangefinder is simple, it’s an optical triangulation device coupled to a cam on a lens. Align the two images in the viewfinder and the camera is in focus. People put a lot of trust into the Leica cameras; the camera works smoothly and the viewfinder and rangefinder are bright and clear and focus onto infinity – in fact the rangefinder unit contained within is probably the most complex of any camera.

Coupled with some very expensive lenses, a Leica rangefinder camera should be capable of producing some very sharp shots shouldn’t it? Not necessarily….

The lens

Within a lens, there two things that are important to ensure correct focus. Firstly, the RF coupling cam should be accurate and second, the optics should be correctly collimated. Generally, it is unusual for there to be any error in the RF cam. In other words, when the lens is set to infinity, the position of the cam usually agrees but it can be out occasionally.

What I do find quite regularly though, is that some lenses focus a little past infinity. This is a problem with an RF camera because shooting at infinity with such a lens, no matter how good it may be, at any aperture will produce exceedingly poor results. At closer distances this shouldn’t be a problem but it’s worth mentioning.

On lenses with goggles there is an additional complication. The goggles themselves have to be precisely calibrated. I was checking a goggled lens earlier on today with a camera with a good rangefinder. At infinity all was well but at 1 metre, the rangefinder was 5cm out. This would be immediately spotted on an SLR but one would not expect that of a Leica. I had to drill out and replace a grub screw to fix it.

The camera

As far as the camera goes, many people know how to adjust the rangefinder within a Leica M camera. You have the large slotted screw on the rangefinder arm which is on a concentric. This allows for horizontal adjustment. Behind the small screw near the illuminator window is a screw that allows vertical adjustment (later cameras need a special tool for this). Most repairers or the DIY owner will make the horizontal adjustment for infinity but on ‘M’ cameras there’s a third adjustment that I call linearity.

When the camera has been correctly set up, there is an imaginary line on a graph that intersects the 1m point and infinity. If this setting is wrong – as it usually is, the rangefinder may be perfectly set to infinity but as the subject gets closer to the camera, accuracy diminishes at an increasing rate. The third adjustment is found just under the smaller screw that attaches the RF arm to the camera.

A number of iterative adjustments are required between close focus and infinity to get this right. So what? You may say. In most cases it probably doesn’t matter too much, but when so much has been invested in a camera and probably even more for the exotic lenses that go with the system, an inch or two focus shift when focussing on the sitters eye could leave the photographer very disappointed with the result.

With something like a SLR, what you see is what you get. It’s extremely unlikely that the focussing screen would be out of alignment and even a badly collimated lens would be automatically corrected by the visual focus confirmation in the viewfinder.

The conclusion

So, the lens cam has to be right. The lens collimation has to be right. The rangefinder needs to be correctly adjusted so that the camera accurately focusses through infinity and down to 1m and below. Any lenses with supplementary goggles need to be correctly set in addition to the lens itself. I suppose in most cases these things go largely unnoticed. General photography, street photography etc where precision is less important than ergonomics.

It reminds me of a comment made by Leitz management during a visit by British and American officials, post WWII to Wetzlar, where it was noticed that slow speeds on production line cameras weren’t particularly accurate. Leitz acknowledged this but pointed out that “however, … the results obtained were quite good enough for all general requirements.”

I don’t wish to portray this as a bad camera, not by any means. The trouble is that like a Ferrari, everything has got to be just right in order to get the performance and there are many small things that can go wrong that wouldn’t be obvious when using the camera. Of course, this isn’t blaming the camera. Correctly set-up, all should perform well, but the nature of the system means you have to put your trust into it.

Thanks, Alan – CameraWorks-UK

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