KoNickon said:
Unfortunately none of the fixed lens rangefinders has a "hard edged" RF patch like that of the Leicas, Bessas or the Hexar RF. (Why is that? Too expensive to produce?) A big part of the struggle with using these fixed lens cameras is finding the RF patch, especially in low light and/or with a cloudy viewfinder..
KoNickon,
I include after this message a very informative post from Jim Williams initially posted on the Rangefinder mailing list (I got the quote from
Robert Monaghan's Medium Format Megasite, the reference about medium format). After reading this post, you will understand why so many cameras (including Nikon rangefinders) have a fuzzy rangefinder patch.
Cheers,
Abbazz
Date: Tue, 23 Mar 2004
From: Jim Williams
jimwilliams1@cox.net
Subject: Re: [RF List] RF tech question
marcus wrote:
> How come most rangefinders have the fuzzy edged RF
> spot compared to Leica and Cosina which have the super
> bright straight edged spot? Is it a much more complex
> accomplishment or what?
Yes, it's much more complex. The recipe:
-- For a basic rangefinder optical system (no viewfinder) all you need
is a fixed, diagonal semitransparent mirror (aka beamsplitter) and a
pivoted fully reflecting mirror (or prism.)
-- For a basic viewfinder (of the usual 'inverted telescope' type) all
you need is a negative lens at the front, to 'minify' the view so it
corresponds with the angle of view of the lens, and a positive lens at
the back to help focus your eye on the image from the negative lens.
(If you didn't mind the camera being 10 or 12 inches thick, you could
dispense with the positive lens entirely, and just have a peep sight to
line up your eye correctly behind the negative lens.)
-- To combine these, all you have to do is stick the beamsplitter
between the negative and positive lenses. Then, to make the rangefinder
image and the viewfinder image focus in your eye at the same distance,
you add another negative lens of the same power somewhere between the
beamsplitter and the rangefinder image. (Sometimes it's in front of the
rangefinder mirror, sometimes it sits crossways in the middle of the
light path -- doesn't matter as long as the viewfinder image and the
rangefinder image have the same total diopter.)
Now you've got the traditional "fuzzy spot" range/viewfinder. The edges
of the RF spot are fuzzy because the edges of the RF window are much
closer to your eye than the apparent distance of the actual viewfinder
and rangefinder images -- so, your eye can't focus on both the edges of
the spot and the actual image at the same time. That's not aesthetic,
but it does make it possible to design a very effective
range/viewfinder without a lot of parts and delicate adjustments.
Now, though, think what happens if you want the viewfinder image, the
rangefinder image, AND the rangefinder patch edge all to come to focus
at the same distance to your eye. Now you've got to have the negative
lens at the front, as before; the positive lens at the back, for the
eyepiece; the extra negative lens to bring the rangefinder image to the
same focus; PLUS a mask for the rangefinder image, to define the sharp
edge, and an additional lens with a hole in it, so it focuses the mask
image WITHOUT affecting the focus of the rangefinder image passing
through it. Again, the trick is to make all the total diopters add up
-- but now you've got to balance three numbers (viewfinder image,
rangefinder image, rangefinder mask) instead of just two.
Add in a parallax-compensated projected frameline, and now you've got a
FOURTH thing to focus, so now you need an extra mirror (to reflect the
framelines into the viewfinder path) and an extra lens (to bring them
to the same plane of focus as everything else.) For an even higher
degree-of-difficulty rating, add a meter readout into the mix!
Of course there are lots of variations on these themes. For example,
Canon's switchable-magnification finders put the whole rangefinder
optic system, including the beamsplitter, in FRONT of the negative lens
for the viewfinder; they pulled off this trick by using a beamsplitter
that was semitransparent only in the middle, for the rangefinder spot,
and clear around the edges. Doing it this way let them switch
viewfinder magnifications (via a rotating optical box) without having
to switch rangefinder magnifications at the same time -- since the
rangefinder was out front, it didn't care what was happening behind it.
But if you look at the basic problem -- bringing objects at different
distances to the SAME focus in your eye -- you can see that the more
things you're trying to focus, the more lenses you need and the
trickier it is to design the system. That's one reason high-end RFs are
more expensive than SLRs with the same mechanical specs
