[Leica] RF accuracy

["Erwin Puts" <imxputs@knoware.nl>]

I have been on holiday for a week, so I could not respond on a timely base.
The resolving power of the eye is measured in angular degrees and is
dependent on the absolute distance of the cones. This absolute measure is
being affected by a number of factors, like age, ambient light, contrast,
fatigue and the visual form of the object under investigation. That is why
resolution tests are so notoriously unreliable. As most people cannot handle
angles and degrees, the resolving power of the eye is often defined in
relation to a fixed distance, mostly 25cm. But the resolving power differs
greatly between the form of the object, ranging from  5 seconds of arc for a
stereoscopic vision and 5 minutes of arc for the distinction of letters,
like an 'E'or an 'F'. For photographic purposes the point source distinction
is used and for RF accuracy the vernier acuity. Both differ by a factor of
6. When discussing resolution of the eye, most students use an average of 2
minutes of arc. Translating this to the ability to distinguish between lines
of a certain width is difficult and here we see figures of 3 lines per mm to
20 lines per mm. Most students and optical designers settle for an average.
In my calculations, (see website) I have used both the average numbers and
the acuity numbers, giving an average and an optimum figure.
It is obvious that any enlargement will improve the eye's ability to
distinguish between objects, as such an enlargement will improve on the
angular measure. That is why an SLR focussing screen wins hands down when a
focal length of more than 100mm is used.
There is nothing wrong with the standard calculations of accuracy of the
rangefinder base. And the accuracy will improve when the enlargement factor
is increased. If you would use a magnifier eyepiece of factor 2 on a M6, you
will get a more accurate rangefinder spot. Note the old attachment to the
135mm Elmarit. This just enlarged the viewfinder magnification and thus the
effective rangefinder base.
The Effective Base Length is a good measure of the rangefinder accuracy. How
much accuracy we need, is a different story. See age, fatigue, ambient
light, contrast, etc. Most RF figures are based on the average figures (see
above), BUT also on the expected amount of enlargement, on the defocus blur
and on  the depth of field calculations, that use a figure of 0.03mm as a
baseline.
Using the vernier acuity as a base, the .58 finder will focus the Noctilux
to any required level of accuracy, given the DoF calculations and the normal
eye and normal ambient light levels. Of course, when in very dim light, low
contrast, loss of a night's sleep and full of adrenaline when taking
pictures of objects that excite the photographer, some loss of accuracy may
be expected. And the 0.85 is easier to focus, if not more accurate.

Erwin