[Leica] Viewfinder parallax question

[Alex Hurst <corkflor@iol.ie>]

Hi LUGnuts

I've had a most interesting session today deciding which of the various
v/fs I have would best match the 80/4 Nikkor lens I have acquired with
Nikonos V camera. This is an amphibious lens - you can use it either above
or below the surface, and naturally I'm talking about dry land v/fs for the
present.

The choice was:

1) Russian turret finder (use 85mm)

2) Komura variable v/f which was bundled with the Komura M 2x extender.
(80mm exactly).

3) Leitz 90mm b/l

The main problem is parallax, because the Nikonos has a high built-in
finder (35mm) sitting directly above the lens. It probably adds about an
inch to the normal position of an auxilary finder on an M Leica.

I rejected the Komura, because apart from poor contrast, something centred
horizontally in the camera's v/f was shown considerably off to the left in
the auxiliary.

The Russian turret v/f was OK - 35mm field of view coincided almost exactly
with the built-in finder, and a centred horizontal object was the same in
both. The problem is that to compensate for parallax on this v/f, you move
the lens turret itself, and the gradations are very small with no
click-stops.

Which leaves us with the Leitz 90mm b/l. Excellent image, a long
compensation scale for parallax fine-tuning, and a bit of leeway because
the 90mm shows considerably less than you actually get.

So the Leitz b/l is my choice, and I've taken some shots to calibrate it ,
using a rough rule of thumb that the extra inch away vertically from the
lens axis probably means that you have to compensate around twice as much
(i.e. if the subject distance is 15 feet, set the v/f compensator at 7.5
feet.)

While I'm waiting for the shots, I'd be interested to test the theory of
parallax, which is fairly simple math, but which I've sadly long since
forgotten.

My question is, relative to the position of the Leitz 90mm finder on a M,
what is the theoretical factor I should divide into the v/f compensation
scale when it's an inch higher? Remember I'm dividing by two as a starting
point, but that's very rough and ready.

Over to you gurus.

TIA

Slan

Alex