[rollei_list] Re: OT: Back Focus and TLR/SLR Lens Types (was 2.8F opton?)
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 7 Jul 2008 19:22:13 -0700
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, July 07, 2008 5:48 PM
Subject: [rollei_list] OT: Back Focus and TLR/SLR Lens Types
(was 2.8F opton?)
OK, Carlos has supplied a link which verifies that the 5
element
80/2.8 Biometar is common between a MF SLR and a Rollei
TLR. You are
saying that the BF for the Planar/Xenotar/Biometar 80/2.8
5 element is
common at about 80% of FL.
Yet we know that the Planar 80/2.8 is not a common design
between
Rollei TLR and Rollei SLR (or Hasselblad SLR). So the
Praktica can
accommodate a lens which the Rollei and Hasselblad SLRs
cannot. Carlos
wonders if it is the difference in shutters. I wonder if
it is a
difference in reflex mirror designs.
Any thoughts?
Eric Goldstein
Its possible. There was a 1930s SLR, not sure if it was
the KW or not, that had an arrangement where the mirror
hinge slid backward a little as the mirror came up. That
allowed the use of physically longer lenses than is usual
for an SLR. I have no idea if this was ever used in a modern
120 or 35mm SLR. Without a schemtatic of the the lenses and
maybe other data its pretty hard to know if the SLR version
of the Biometar was made retrofocus but it may have been. I
suspect it would need more than five elements to do it but I
am not a qualified lens designer.
I think also that because the mirror dimention does not
have to be the full size of the frame it may be possible
that it clears a standard type lens of "normal" focal
length. Remember, the mirror is midway in the optical path,
the swinging part being closest to the lens where the cone
of light from the lens has not spread out yet, so it does
not have to be the full size of the film. My Nikon is like
this where my Graflex has a larger mirror mainly because it
has a rotating back, that is, the mirror must accomodate the
larger vertical dimension when the back is in the vertical
position. This isn't necessary in a camera with a fixed back
and the long dimension of he film perpendicular to the
direction of mirror movement.
Since Exakta was mentioned I will point out that the
lens most often used on these at one time was a 55mm Biotar
but the longer FL is evidently not necessary because a
number of conventionally designed 50mm lenses were also
available for it. For instance I have a 50mm Zeiss Pancolar
on mine. Again, I think the radius of the mirror is smaller
than one would think.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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