Re: [Leica] Rangefinder Terminology

[John Collier <jbcollier@home.com>]

A rangefinder is able to measure the distance from an object to the observer
by measuring the angle a prism or mirror is rotated to superimpose two
images from points perpendicular to the object. There are three variables:

1) the quality of the mechanism. The Leica rangefinder does all its work
with only 4.5mm of arm travel! This allows the mechanism to be very compact
but also means it has to very carefully built and calibrated.

2) the magnification of the two images you are lining up. The more
magnification, the more precisely you can match the two superimposed images.
The ability of the operator's eye to resolve subtle differences has a strong
influence as well. This why some people are fine focusing wide aperture
lenses even with low mag finders such as the Hexar RF (0.60x lifesize).

3) the distance apart the two images are sampled from. The larger the base
of the rangefinder, the easier it is to register subtle distance differences
in objects far away. This is easy to demonstrate by looking at a view with
objects at different distances. Shift your head side to side slightly.
Notice how objects close to you shift from side side much more than objects
quite distant from you. The further you shift from side to side, the same as
increasing your rangefinder's base length, the more distant objects increase
their small shift, making it easier to judge subtle differences in the shift
of distant objects.

The plane of focus of a lens is just that: a flat plane. There is an area in
front and behind the plane of focus that appears sharp due to the small size
of the "circles of confusion". A line of points running perpendicular to the
plane of focus, both ahead and behind it, will be rendered as small circles,
called "circles of confusion", whose size depends on three things:

1) the size of the lens aperture. Small apertures make the out of focus
points register as smaller circles of confusion than at large apertures.

2) the distance the points are away from the plane of focus. The further a
point is away from the plane of focus the larger the circle will be.

3) the magnification or size of the object in the image. The smaller the
point is rendered, the smaller the various circles of confusion will be. A
wide angle lens will render a particular point at a specific distance
smaller than a narrow angle lens (telephoto) will.

So we know that small apertures give us small circles of confusion which
makes the image seem to have great depth of focus, things both close and far
appear sharp. Larger apertures do the reverse where only a narrow area is
sharp in the image.

Now lets put it all together! The whole idea of the rangefinder is to place
the object you want focused in the image within the depth of focus of your
lens. It is much harder to do this with a large aperture lens, wide open, so
we use the longest based, highest magnification rangefinder we can. Simple!

Here is a website that explains it all with a little more technical jargon
but, to its credit, has a picture!

http://www.imx.nl/photosite/leica/m/bodies/rfaccuracy.html

and another picture off of the Leica homepage:

http://www.leica-camera.com/kultur/mythos/innovationen/messsucher/index_e.ht
ml

Now to your specific questions. The M3 has a higher magnification finder
(0.91x lifesize) than the M2 (0.72x lifesize) so focusing is easier. I like
the 0.72 finder as it has the best 35mm frame and that is my most used lens.
The high magnification finders have a 35mm frame that is hard to see or, in
the case of the M3, it does not have a 35mm frame at all. I also have
excellent eyesight so I do not need the extra magnification of the M3 or M6
0.85.

I hope this helps but I sure wish I could have included pictures! Please do
post any questions no matter how simple you think they are. That way you, I
and others, who are too shy, can learn more.

Your circularly and confused,

John Collier


> From: "Tony Salce" <NadinaTony@bigpond.com>
> 
> For someone reasonably new to rangefinders, could someone be so kind as to
> explain in simple english, what is meant by the following quote from Dick
> Gilcreast's article in the latest edition of Viewfinder, in respect of his
> preference of the M3 over the M2:
> 
> " In a rangefinder camera the RF needs to have enough base lenght in order
> for the indicated focus point to be WITHIN the rather narrow depth of field,
> so it made sense for Leitz to design their original M camera around these
> two lenses." (ie 50mm and 90 mm- this is my addition)
> 
> Could you also explain why the 0.72 magnification is not as good for the
> 50mm lens as compared with a 35mm lens ? I hope that question makes sense.
> 
> Actually I don't fully comprehend ( I am embarassed to admit) the
> distinction between the 0.92 magnification of say the M3 and the 0.72 of the
> M2.
> 
> Your reply would be appreciated.
> 
> Kind Regards,
> 
> Tony Salce
>