Re: [Leica] Rangefinder Accuracy

[Dennis Painter <dpainter@bigfoot.com>]

Rod Fleming wrote:
> 
> 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.)

The angle of movement depends upon the baselength. Two rangefinders with
the same baselength but different magnification will have the same angle
of movement. This is simple geometry.  The one with greater
magnification will be more acurate since it allows the human eye to more
closely discern concidence of the images.

> 
> 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!

You are only looking at part of the linkage. That rotation of the Contax
still has to be translated to the same 0.1 inch of linear movement of
the lens in order for the focus to be accurate. All of the linkage from
the linear movement of the lens to the rotational movement of the mirror
must be taken into account.
> 
> 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?

Interesting yes, and I much prefer the M also.

> 
> Just thought I'd throw that in to provoke thought
> 
> Cheers
> 
> Rod