M8 part 8: Rangefinder accuracy
Background
Rangefinder accuracy in the sensor based camera
Rangefinder accuracy is the result of several factors: the mechanical accuracy of the camera/lens coupling in combination with the rangefinder unit alignment on the camera, the focus shift of the lens in question when stopping down, the accuracy of the location of the focal plane (film gate or sensor surface) related to the bayonet location and not to forget the quality of the eye of the photographer.
The mechanical accuracy of the lens/camera coupling is the result of the match of the shape of the roller and the shape/adjustment of the roller arm (prism arm) and the shape/steepness of the lens cam. Location of film gate and bayonet and the alignment of the rangefinder unit on the body are also accurately fixed. The accuracy is needed over a large range of distance settings from 0.7 meters to infinity and some machined corrections are necessary to guarantee the accuracy over the focus range, the close range being especially critical. Leica therefore adjusts the rangefinder settings at 1meter and 10 meters.
With film based cameras the adjustments can be done within an accuracy of several hundreds of a millimetre (0.02 mm is cited as film gate tolerance). This is very accurate given the fact that the film itself has a certain depth of emulsion (normally around 20 micron) and has always a certain curvature itself. The camera and lens designers assume a certain location were the optical focal plane of best performance will be located. That is the position where the focus plane will be located when the rangefinder spot is accurately aligned. Some latitude is acceptable here because of the emulsion thickness and the film curvature. Both phenomena will ensure that the focus plane will be located inside the emulsion layer and will generate a sharp image.
Film based cameras
Assuming that the mechanical parts are accurate, we can be sure that the plane of sharp focus is there where the designers want it to be. But we know that the any lens has some degree of spherical aberration (SA). This implies that we do not have a fixed point of sharp focus, but we have some latitude in locating the image plane. SA will force rays form the outer parts of the lens to focus at a different location than the central rays do and we will have the situation where rays from a point will at first converge to a small circle and then diverge again. This small circle is of course related to the circle of least confusion. We know that the smallest circle will consist of a small patch of concentrated light with a larger halo of diminishing intensity around it. Here we find the best resolution. But just before and after this location we find a circle of light that is a bit larger but has less halo around it. Here we find a pont with maximum contrast and somewhat lower resolution. The designer of the lens has to decide where to locate the focal plane and thus the image plane of best performance. But wherever that location is, it is designed for the wide open performance of the lens. When stopping down the rays at the edge of the lens are cut off and the focal plane starts to shift: this is the phenomenon of focus shift that I discussed for the first time when assessing the Noctilux performance. It has been the first time that attention was drawn to the phenomenon of focus shift. In practice that implies that you focus for wide open performance but get the location of the sharpness plane for a smaller aperture. For critical work that can be unpleasant.
These lens characteristics cannot be changed. Zeiss has claimed that their lenses are designed to exhibit less focus shift than is usual, but part of the claim is based on a different location of the focal plane: a kind of average position so to speak. Not as good for best wide open performance, but better for stopped down performance. Leica has adopted the opposite approach: optimize for wide open performance and let the focus shift be compensated by depth of field.
Sensor based cameras
The main characteristic of the sensor surface is its absolutely flat surface: there is no depth like we have in film emulsions. Here we cannot count on emulsion thickness to compensate for mechanical errors in accuracy or focus shift.
Leica does know this of course. And the tolerance level of the whole rangefinder adjustment chain has been narrowed accordingly. The tolerance level in film based cameras is a few hundreds of a millimetre, in the M8 that level has been reduced to a few thousands of a millimetre. That does not imply that the factual tolerance level is a factor ten narrower! In reality the factor is about three to four times. This makes the M8 the most accurately machined and assembled M camera in history. This is done to compensate for the lack of image capture thickness.
When focussed accurately the focus plane is spot on on the sensor surface.
The other side of the medal is the fact that focus shift might be more noticeable when stopping down: another argument not to stop down too much when you need critical sharpness at the focus point.
We should also realize that any mechanical part and any manual adjustment has a small level of tolerance that may cancel out (good) or add up (bad). So some misalignment of the rangefinder mechanism should be kept in mind. You cannot work at zero tolerance. Here we run against the limits of mechanical precision equipment.
The test
To find out how narrow the tolerances are, I conducted the following test. I took a picture of the Siemens star target at 2.5 meters distance and focused with the magnifier on the vertical black bar of the chart. With this bar we can optimize the accuracy of the vernier alignment. The actual thickness of the bar is one millimeter.
Then I did a series of pictures of pictures wth focus bracketing to ensure that this is indeed the plane of best sharpness. Then I deliberately shifted the focus by the width of one millimetre (or thickness of the bar) and took again a series of pictures with focus bracketing. The result you see below. The misaligned picture has reduced contrast and reduced resolution. All pictures wide open to evade focus shift problems. Left: in focus; right out of focus by one millimeter.
The small shift in focus inaccuracy is really small: at this distance of 2.5 meter the shift by one millimetre amounts to a focus displacement of 1/43 of a degree or roughly one second of arc. The whole movement of the lens covers three degrees and this small misalignment will happen almost always: you are in a hurry; you cannot focus accurately because of handholding movement or because of eye fatigue etc.
When you encounter unsharpness where you expect a sharp image plane you may have to think about operator problems first (eye fatigue, hand movement), then focus shift (when stopping down) and then about possible mechanical tolerance issues. A good test is to do focus bracketing (cheap because digital files do not cost a cent) and to repeat the test several times over a week to see whether there are operator induced errors.