[Leica] M3 repair (suite), collimation mire drawing

[Grégoire Vandenschrick <vandenschrick@geog.ucl.ac.be>]

Hi, two parts in here: first my adventures in the M3 viewfinder, and second,
the math behind my drawings to test the collimation of the telemeter:

first
here is the situation.

I know a lot of you fear to open their M, so not a reading for the fainted
heart ;-)

after having tried to file a screw at exactly the right dimension of the
screw of the rewind knob of the M3 first generation (not one red hole, not
two red holes) and using it without any succes, I suddenly had a flash (lol)
why not unscrew all the rewind knob from its pillar. This was luminous and
worked very well (easier to unscrew, because of the easy grip on the rewind
knob, though you have to block the rotation of the pillar wich is perfectly
round and flat). So I managed to lift off the upper carter (at 2 O'clock
this morning, my little daughter was sleeping between two meals, 19 days
old!, and the mother was sleeping too, good timing so).

Boy, this is a rugged viewfinder you find then, very few glasses surfaces
are accessible, as a lot is thightly kept in place by metal housing. My plan
was to clean all the free glass surfaces, so I made that, where I could.

Well, not a lot of improvement, the frames and the telemeter windows look
still yellowish tinted. There is less dirt in the frames, as I blew some air
with a little rubber pump for lenses on the occulted glass where the frames
are drawn.

I noticed something, and would like to know if this is normal or should be
fixed (surely not by me, I don't thnik I have the ability for this) the
picture of the telemeter is driven in the middle of the glass frame by a
tiny roof prism, on wich two tiny striated planes are glued, one on the top
of it (looking in the lensedirection), one on the bottom of the prism
(looking so in the frame glass). These seem to be unglued a little bit, as
there are air bubbles between these tiny rippled planes and the prism. What
is the use of these planes first and should I have to have it reglued in
place, but this woudl need to have this prism extracted from the path (a
tough work not for me)?


second
I also trigonometrically calculated and drawn some collimation mires (at
infinity, at 10m and at 1m) to be placed at 2m and 3m behind the film plane,
based on the 69.25mm base lenght, and compared my M6TTL with the M3. They
both seem very accurate, vertically and horizontally. So, the only problem
is this soo dim telemeter patch in the M3.

If you are interested by this, here are my calculations, very simple, maybe
wrong:

- -the telemeter measure base is 69.25mm broad, let call the center of the
main window A, and the center of the telemeter window B (distance between A
and B is 69.25mm so).
- -Assume we make the point on a subject at a distance X,
- -and the mire sheet is placed at a distance Y (Y beeing less than X)

First case, the infinity, the easiest:

whatever the distance between the film plane or the camera and the mire, the
two windows are looking straight, the rays coming from the point A and B are
parallel. So you just have to draw two symbols separed on the sheet by
69.25mm.

the other case (10m, 1m), simple in fact:

when focused to let say 10m, the two rays coming from A and B are looking at
the same point X. the ray coming from A is still normal to the plane of the
telemeter base, this is the ray coming from B wich is rotated towards X
(left) by the rangefinder system and the lense. This ray coming from B
intersect the mire plane (let call this point b) at a certain distance (let
call it d) of the intersection between the ray coming from A and the same
mire plane (let call this point a).

So, we have two similar triangles: ABX, with base lenght = 69.25mm and
height = X (10m, 1m), and abX, with base lenght = d and height = X-Y

so, to know the d distance to draw symbol to superimposed view for an X
distance is easy:

d = 69.25 * (X-Y)/X,

Y beeing the distance you put between the camera and the mire plane.

I didn't check if the view was really sharp with a groundglass put on the
film guide, but I assume my M6 to be sharp (it is) and made the test with
the same optic (50mm summicron new, but this has no importance) and checked
if the point was correct for the read distance on the barrel of the optic
(exactly at the same place for infinity, for 10m and for 1m)

A reflexion on the precision: with a good drawing program (I used
powerpoint, wich is not very accurate, but Corel Draw, or AutoCad should be
alright) and a good laser printer, you can obtain really fine crosses. Now
about the Y, the more big on this, the better. If the mire is closer to the
camera, the error of drawing, parallelism between camera and mire, and the
error on Y will be relatively more important on the angle, lessening the
accuracy of the test, so if you can draw your mire to be placed at 5meters
or more, this is better.

A remark, If Y is bigger than X, the system still work, but the mark to be
seen by the telemeter window will be placed left from the mark beeing seen
by the main window.

That's it, sorry to be soo long and boring, but I wanted to share this, as I
had fun making it and testing, maybe you too could.

Let me know if something is completely wrong with my arithmetics.

Best,

Greg VDS



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