Film Plane Register


Intro.
There is still considerable confusion about the now famous difference in register between the Konica Hexar RF and the Leica M. There are a few issues that need to be comprehended when discussing this topic. Let us start with the basics. Below is a diagram, drawn by me to illustrate the dimensions and mechanics.
Explanation
The guide rails and the register rails define the film channel, which has a universal depth of 0.2mm, with some tolerance of ± 0.01 to 0.04mm. The Leica dimensions are as follows.
The distance from bayonet flange (lower part in the diagram to the pressure plate (or guide rails) is 27.95mm.


The distance from bayonet flange to the register rails is 27.75mm. The film channel depth is a fixed dimension and cannot be changed.
The film thickness is somewhere between 0.13 and 0.15mm, with most films at 0.13mm, so let us settle for 0.13mm. The emulsion thickness is much less, and faces the bayonet flange, where the lens is attached.
The back focal distance of the lens is officially the distance from the last element of the lens to the focal plane, that is the plane where the lens focusses its rays. It is also called the sharpness plane or the plane of sharp focus. In this diagram the back focal length is the distance from the sharpness plane to the bayonet flange and for Leica it is 27.80mm.
Where is the sharpness plane and the emulsion layer of the film?
If the film were flat and the emulsion layer is 0.15mm from the pressure plate, the lens would image the scene exactly in the emulsion layer. The film emulsion is at a position 27.95 - 0.15mm = 27.80mm. In the ideal world, the sharpness plane would coincide exctly with the emulsion layer.
Let us now introduce the concept of depth of focus. That is the amount by which an image plane may be shifted horizontally with respect to a reference plane (film emulsion or tracking rails or back focus) before the image blur becomes unacceptable.
The depth of focus is defined as the blur circle times the aperture times 2. For a 1.4 lens and a really exacting blur circle of 0.017mm (1/1500 inch), the depth of focus is 47 micron. So the location of the emulsion layer may shift by this amount before a change in contrast or sharpness can be detected.
With an f/2 lens the depth of focus is 68 micron or close to 0.07mm.
Film is not flat and mostly bulges forward. Measurements by Zeiss and Kyocera indicate an average displacement of 30 to 40 micron (0.03 to 0.04mm). So depth of focus covers the bulging of the film, even when using exacting standards.
Tolerances.
It seems logical that the bulging of the film is incorporated into the design and the necessary dimensions. Leica repair manuals inicate that the toerance on the minus side is higher than on the plus side. It is safer to have the film at a location closer to the bayonet flange and the back focal length as then the sharpnes spalne will always slide through the bulging film. If we would allow the film to be located away from the bayonet flange, the sharpness plane may not or just touch the front of the bulging film and a greater amount of unsharpness would be the result. The table below gives the figures. Register can be from 27.96 to 27.93. Film plane (assuming a 0.13mm thickness) can be from 27.83 (flat) to 27.77 (bulging forward). Back focal distance has some tolerance too (±0.02mm).
    

LEICANominal
From
To
Register
27.95
27.96
27.93
Film plane flat
0.13
27.83
27.80
Film plane bulging
0.16
27.80
27,77
BFD
27.80
27.82
27.78

So we can see, that even in the worst case, the sharpness plane is 0.01mm off from the film plane with bulging and this difference is well within the depth of focus of 0.04 to 0.05mm.
What is the cas with the Konica Hexar RF and Leica lenses? The table below gives the figures. Now we see, that in a number of cases the depth of focus will not cover the tolerance margins. If the Leica lens has spot on BFD, then the chance is very high that the Konica body will be outside tolerances, including the depth of focus.

    

KONICANominal
From
To
Register
28.00
27.97
28.03
Film plane flat
0.13
27.84
27.90
Film plane bulging
0.16
27.81
27,87
BFD of Leica lens
27.80
27.82
27.78

Even when there are zero tolerances, the film emulsion will be at 27.84mm, where the lens will focus at 27.80mm and so just outside the depth of focus.
All of this has been based, of course, of critical work. At handheld shooting with ISO400 and aperture 4.0 or smaller and a print size of 20x25cm, it will depend on your own standards of performance, if you will detect the loss of quality.
Also the accuracy of the rangefinder, the mechanical tolerances in the roller cam mechanism, etc wll influence the results.
Generally we can assume that at least a significant proportion of Hexar-Leica combinations will be outside the required tolerances for critical work. But in a substantial number of cases, the loss of quality will be acceptable.
Anyone has to decide fo him/herself what is acceptable.