The Rangefinder is a rather simple operating
object. Somehow when you turn the focus wheel on top, something
magical happens to make two images converge in the viewfinder. As
with every magical trick there's always a slight of hand and here's
a couple here to consider.
First, there is the glass prism/beam splitter,
which in this example with a laser pointer shows it doing the job.
The other part to the assembly is the rangefinder compensator which
redirects the moving image or as in this example the right most beam
The rangefinder compensator consists of a
moving plano-convex lens (1), a fixed front plano-concave lens
(2) and compensator lever (3)
Turning the focus wheel (10) activates a series of gears
(9&8) that meshes with the lens barrel helical at (7). Gear (8)
has a worm spiral (6) on its shaft that meshes with a worm gear (5)
that is connected on a common shaft to actuate worm gear arm (4).
contacts a cam surface on compensator lever (3), to move the segment-shaped plano-convex lens
element (1). The positioning of the plano-convex lens element (1) in relation to the fix front plano-concave lens
(2) is what
redirects the moving image as seen in the viewfinder.
Here's the rangefinder compensator in operation with a bunch of stuff removed to show the movement starting from infinity through to .9m setting. The infinity position is when the plano-convex lens is rotated all the way to the right and conversely .9m has the lens rotated all the way to the left.
Pretty simple eh?
Except here's the rub, the position of the plano-convex lens is directly related to the rotational position of the lens barrel helical.
To see how complicated this concept gets, visit
November 11, 2008