[Leica] Rangefinder Accuracy

["Rod Fleming" <rodfleming@sol.co.uk>]

Hi

Lots and lots has been said about the relative accuracy of rangefinders, and
most tends to discussion around the equation

base length x viewfinder magnification = Effective Base Length, or EBL

EBL is then taken as a measure of accuracy.

Surely this is only the case if we are comparing two rangefinders of the
same design, though?

It seems clear to me that other factors have a bearing.

The first would be the angle that the moving prism or mirror moves through
between any two focus points. For example, say the moving prism turns
through an angle of 5 deg. between infinity and 10 feet- surely a
rangefinder in which the equivalent movement is 10 deg would be twice as
accurate? (I don't know the specs for the various Leicas-but I can't see how
that affects the principle.)

And then, are we not talking here about ease of focus, rather than accuracy
of focus? The latter is surely an absolute term, within a tolerance (which
we can specify) - the former raises questions about the user's vision among
other things, like the brightness of the RF image etc.

Another thing. Look at the Contax style rangefinder. This tranfers
rotational movement of the lens barrel directly to the rangefinder via
gears. The rotational movement of the lens, between infinity and 1 metre, is
about 3/4 turn on a diameter of just over an inch- say 2.5 inches. The same
range of lens extension is transferred to the Leica style rangefinder by a
cam moving approx 0.1 of an inch!

Am I being thick or does that mean that a Leica type rangefinder has to be
engineered to an accuracy at least  _twenty-five_  times that of the Contax
type in order to achieve the same overall accuracy? (And I do mean accuracy,
not ease of focus.) And if it doesn't, would somebody please explain why it
doesn't?

As a matter of fact I find it a lot easier to use the Kiev 4 (a Contax copy)
rangefinder than that in a Leica IIIa ( I happen to have one of each of
these to compare- I'm sure other comparisons could be made)- even though the
viewfinders are just about equally dim and squinty. But that's personal- and
neither of them, needless to say, match the M3.....But it's interesting, no?

Just thought I'd throw that in to provoke thought


Cheers



Rod