[rollei_list] Re: WTB: SL66 thingies
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 22 May 2006 18:39:51 -0700
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, May 22, 2006 4:50 PM
Subject: [rollei_list] Re: WTB: SL66 thingies
I think I'm hearing an echo? :-)
The SL66 is a retrofocus type. The TLR Planar isn't.
Neither of these
lenses is symetrical, so that particular point does not
apply in this
instance...
Eric Goldstein
Perhaps there is a misunderstanding about what
"symmetrical" means in a lens. The TLR Planar and Xenotar
are not perfectly symmetrical but are essentially so despite
the odd number of elements. If one considers the cemented
component in one side as a single element the symmetry
becomes more clear. A lens like a Tessar is not symmetrical
because the two halves have different signs. The front
combination is negative, the rear positive, so there is no
cancellation of lateral aberrations. In a lens like the
Xenotar both halves are positive so there is some
cancellation. Lenses with similar construction but different
power in the two halves, like the familiar Biotar, are also
essentially symmetrical. Actually, the shift in power from
exact symmetry is made to try to match the shift in symmetry
of the object and image distances and improve the symmetry
of the entire system where object and image distances are
not equal.
Because of the strong negative element introduced at one
end in either a telephoto or reverse telephoto (retrofocus)
lens the two halves become quite unsymmetrical even though
the "inside" lens of one or the other (more common for
retrofocus lenses) is essentially symmetrical.
Complete symmetry of the optical system, including equal
object and image distances, automatically cancels lateral
color (difference in color magnification), geometrical
distortion, and coma. Even when a completely symmetrical
lens, like a Dagor, is used for objects at infinity, the
cancellation of these aberrations is substantial. All can be
corrected in non-symmetrical lenses but it takes more effort
on the part of the designer and may not be complete. For
instance, The Commercial Ektar, a Tessar type lens, is very
well corrected for lateral color, but still has some coma
and must be stopped down 2 or 3 stops to get rid of it:
something which is typical of Tessar lenses. Coma can still
exist in symmetrical lenses, again like the Dagor, but is
minimal. This is not to say the Dagor is perfect, it isn't
at all, having a set of vices all its own, but those are
aberrations that are not affected by symmetry.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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