Archived posting to the Leica Users Group, 2006/08/30

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Subject: SLR viewfinders (was: Re: [Leica] M42x1-Canon EOS adapter ...)
From: hoppyman at bigpond.net.au (G Hopkinson)
Date: Wed Aug 30 15:48:47 2006

Doug, that's a very comprehensive take on the finder differences.
Up until now, I'd always thought that my MF Nikons (FE2 F2, F3 etc etc)were
great finders vs all of the AF SLRs I've looked through. That is with proper
pentaprisms and the split image (which is what I used the most). I guess
that's still valid, however you've explained that the SL & SL2 are much
better again. Why doesn't that surprise me?
Now you've made me think about another new camera, thanks a bunch. This LUG
is very bad for my bank balance.

Cheers
Hoppy

-----Original Message-----
From: lug-bounces+hoppyman=bigpond.net.au@leica-users.org
[mailto:lug-bounces+hoppyman=bigpond.net.au@leica-users.org] On Behalf Of
telyt@earthlink.net
Sent: Thursday, 31 August 2006 03:50
To: lug@leica-users.org
Subject: SLR viewfinders (was: Re: [Leica] M42x1-Canon EOS adapter ...)

Scott McLoughlin <scott@adrenaline.com> wrote:

> OK, so I give up. Why is it so hard to make a nice SLR VF?
>

A good SLR viewfinder delivers a bright contrasty image that can be seen
clearly over the entire area.  One of the supposed advantages of the SLR is
being able to focus over 
the entire picture area, no?

First you need to deliver a lot of light to the viewscreen, meaning you
need a fully-reflective front-surface mirror, not the semi-silverd mirrors
that AF systems require.  The screen itself needs to transmit a lot of this
light while providing enough of the 'ground glass' effect to focus the
image.  The 'ground glass' effect scatters light, so there's a trade-off
between brightness and the 'tooth' of the ground glass (actually matte
plastic).  AF cameras compensate for the lower light delivered to the
screen by giving the screen less 'tooth'.  You can get a reasonably bright
screen on an AF camera, at the cost of the 'tooth' required for efficient
manual focus.  Nikon makes (or, made) G- and H-series viewscreens for the F
cameras that substituted either clear plastic (G) or microprisms (H) to
make a very bright viewing image.  The microprisms and fresnel rings on
these screens were optimized for particular focal lengths and aperture
ranges.

The viewscreen also needs to include some way of delivering even
illumination; this is generally done with fresnel rings etched into the
screen in addition to a condensing lens on top of the screen.

Next the good viewfinder needs to use a large glass pentaprism, not a
skimpy prism or a pentamirror.  Pentamirrors are light weight but they
don't reflect light as efficiently as a pentaprism.

I'll leave discussion of the viewfinder eyepiece to the optometrists among
us, but here's what Leitz did with the SL and SL2:

The mirror is fully-reflective except for the central region, where the
meter reads the light.  The standard screen in the SL has a coarse
microprism in the central area and a very fine microprism over the balance
of the screen.  The fine microprism is small enough that you can't
recognize it as microprisms, it focusses like ground glass yet transmits
light like microprisms.  The microprisms preclude the use of etched fresnel
rings so instead the pentaprism has a condensing lens ground into the
bottom.  And of course the prism is big to provide good viewing out to the
corners.

The condenser lens in the prism is more expensive to make, the big prism is
heavy, and the fully-silvered mirror means no AF.

> So what about a a nice ~93% coverage , but otherwise spiffy
> VF like in the Nikon FM series?  Why aren't all SLR viewfinders
> at least that good?

in a word, Autofocus.  Try and SL or SL2 and see if you still think the
FM-series viewfinders are all that spiffy.  They're quite good compared
with what most people expect in an SLR viewfinder but compared with the SL
or SL2 there's lot to be desired.

Doug Herr
Birdman of Sacramento
http://www.wildlightphoto.com


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In reply to: Message from telyt at earthlink.net (telyt@earthlink.net) (SLR viewfinders (was: Re: [Leica] M42x1-Canon EOS adapter ...))