[Leica] M Viewfinder Framelines (LONG)

[Dennis Painter <dwp@deltanet.com>]

M Viewfinder Framelines (LONG)

There has been some discussion about frameline combinations lately so I
thought I would make some comments on how framelines are 'controlled' in
the M.  This may be old hat to many here but not all.  One of the
bayonet mount flanges on the lens engages the mechanism that selects the
frameline.  There are three different lengths to this flange which
internally engages the frameline selector when the lens is mounted. 

This corresponds to the three different positions of the frameline
preview lever on the camera. This is a historical feature that goes back
to the creation of the original M-3 finder; however, in the case of the
M-3 finder the 50 frameline was always in view and not selected by the
masks. The 50 frameline always being in view is undoubtly due to the
fact that the M-3 supported internal finder viewing for the 35 (with
goggles) 50, 90, and 135 focal lengths. In this case the 35 with goggles
selected the same framelines as the 50 which is why mounting a 35
without goggles on any M will select the 50 framelines.

Frameline display is controled by masks internal to to the
range/viewfinder.  The masks have cutouts and slight movement of the
mask uncovers and covers the various cutouts to pass light thus
illuminating the selected frameline in the finder.  Since there are
(currently) only three possible mask positions. Thus frameline sets have
evolved over the years from the original M-3.

Lets review the 'length' of the bayonet flange that controls the mask
selected, for want of a better terminology I will call the lengths
"Full, Mid, and Short" to describe the length of the flange.

Full selects the 90 mask
Mid selects 50 mask
Short selects 135 mask

These masks are for the original M-3.  To avoid incompatibilies with
later models the M-2 used the following:

Short selects the 35 mask

Thus the M-2 had 35/50/90 available.  If you mounted the 135 lens on an
M-2 you got the 35 frameline in the finder and had to use an auxilarry
finder to properly frame.  Unless you mount a 135 f2.8 with goggles,
which brings up the 90 frameline and magnifies it 1.5 times with the
goggles to give the same field of view as the 135 lens.

When the M-4 came out the decision was made to include marks for the 135
along with the 35 at the same time, thus begining the trend to multiple
finder images in the viewfinder.  So in the M-4:

Short selects 35/135

With the M-4P and later M-6 it was decided to add lines for the 28 and
75 and the pairing is:

Short selects 35/135
Mid selects 50/75
Long selects 28/90

Modifications to the flange controlling framelines is possible. The most
common modification is to remove additional material on the de-goggled
35mm lens to bring up the 35 mm framelines on an M-2/4/5/6 instead of
the 50 frameline.

Someone recently posted that they had material ADDED to a 28mm Rokkor (I
believe) to bring up the 28 frameline on the M-6.

Trivia:  One of the bayonet flanges was used on the M-5 to control the
arm with the meter cell.  When the lens was mounted the arm would swing
out into metering position.  Dismounting the lens allowed the arm to
swing safely away into its inactive position.  Lenses that protruded too
far into the body to allow the meter arm clearance could be modified by
removing material from the bayonet flange so that the arm did not swing
into metering position when that lens was mounted.  Thus you could not
meter with the lens but you could use it.  I am not sure which of the
flanges controlled the meter arm.

Future:  It would seem possible to us a 2nd bayonet flange to provide
additonal control over frameline selection.  I know, its really tight
inside the finder and the body, not to mention we do need to leave
enough material on these bayonet flanges to hold the lens on!  Another
problem would be how to preview framelines.

I hope this is helpful, are there any corrections or clarifications?

Dennis