[rollei_list] Re: Back-Focus and Retro-Focus
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: "New Rollei List" <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 7 Jul 2008 14:46:38 -0700
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, July 07, 2008 1:52 PM
Subject: [rollei_list] Re: Back-Focus and Retro-Focus
Marc -
It depends upon the design of the SLR. If you will recall
the history
of the early 35 mm SLRs, they featured longer "normal"
lenses
(55-58mm) so that there was sufficient clearance for the
mirror. Then
came the retrofocus versions.
As Richard Knoppow pointed out, a very simple lens can be
designed
with enough back focus to clear an SLR mirror, but lenses
with limited
corrections are not what we are discussing here...
Eric Goldstein
--
My point was only that its possible to move the rear
principle point toward the film without using the specific
design known as a reversed telephoto or retrofocus lens. I
don't think a lens as simple as a single meniscus can be
made to have enough retrofocus effect to clear the mirror of
an SLR at normal FL. Meniscus lenses can be reasonably well
corrected, an example is a single cell of a Convertible
Protar, but the correction of the lateral aberrations (coma,
geometrical distortion, and lateral color) is more difficult
when a lens has no symmetry although it can be done.
I have only a limited amount of patent data in LensView
but the Google Patents search might turn up more. Again, its
difficult to know which patents apply to stuff that has
actually been manufactured. Large companies, who can afford
a good patent department, will patent anything that looks
usable if for no other reason than to keep it from their
competitors.
It is quite possible that a variation of a given lens to
adapt it to a given camera might not be separately patented
in which case the only source of prescription information
would be from the manufacturer.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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