[rollei_list] Re: Back-Focus and Retro-Focus - Flipping lens
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: "New Rollei List" <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 9 Jul 2008 02:58:12 -0700
----- Original Message -----
From: "Mark Rabiner" <mark@xxxxxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, July 08, 2008 10:06 PM
Subject: [rollei_list] Re: Back-Focus and Retro-Focus -
Flipping lens
Are we speaking in ongoing forever generalities or is
anyone specifically
mentioning a specific normal focal length lens which has a
retrofocal
design? And if not why not?
And how can a lens be a "semi" retrofocal design?
If a retrofocal design means in effect a telescope flipped
around how can
you semi flip it around? Such a design seems extreme. You
take an element or
two out of it and call it "semi". I'm not clicking with
it.
mark@xxxxxxxxxxxxxxxx
Mark William Rabiner
Well, I don't know who has been using terms like
semi-retro-focus, not I said the little red hen. In any case
the proper name for this type of lens is reversed telephoto,
the name Retrofocus was originally a trade-mark of Angenieux
for the line of such lenses developed by them. A reversed
telephoto makes use of the effect familiar when looking
through a telescope the wrong way around: it makes objects
appear to be smaller or further away where a telescope in
the right direction does just the opposite. We all know that
if one looks through a negative lens it has much the same
effect. In fact, such negative lenses were once widely used
as viewing lenses by artists and were also used alone as
simple view finders on some cameras. For instance the finder
on early Speed Graphics was a single negative lens mounted
in a mask. By placing such a lens in front of a standard
lens the same effect is produced for the lens, that is, the
projected image appears smaller. However, the distance from
the lens to the film remains the same. In effect its focal
length is shortened but the focus distance remains the same.
It is this effect which explains the use of this type in
single lens reflex cameras where the distance from the back
of the lens to the film must be sufficient to clear the
moving mirror.
The earliest reversed telephoto lenses were for
projection, Kingslake attributes them to Ball and Bowen but
does not give a citation or a patent. The idea was to obtain
a large image when the projector was close to the screen.
One of the first lenses designed specifically to obtain
a long back focus was a 35mm lens designed By Horace Lee, of
TT&H for the three-strip Technicolor camera which had a
large beam splitter prism behind the lens which limited the
minimum focal length possible with a normal type of lens. A
great many reversed telephoto lenses were designed once 35mm
single lens reflex cameras became popular.
Reversed telephoto lenses have some interesting
properties beside the increased back focus. For instance
because the image angle is reduced on the image side they
have less fall off than normal lenses. This may also result
in a difference in the resolution away from the optical axis
but I don't have anything specific about this.
Because reversed telephoto lenses are very unsymmetrical
they must be corrected for the three lateral aberrations,
coma, geometrical distortion, and lateral color, without the
aid of symmetry which provides automatic correction, or at
least minimization of these faults.
Some "normal" lenses have some telephoto or reversed
telephoto properties. Of course, it is the location of the
rear, or second, principal point which determines which, if
either, the lens is. This is the location from which the
image appears to originate and is the distance the focal
length is measured from when the lens is focused at
infinity. The fact is that in telephoto, reversed telephoto,
and some standard lenses, the principal points may lie
outside of the lens. The location depends on the combination
of powers in the lens. It is really the angle of the cone of
light coming from the lens to form the image.
For many standard lenses the principal points are inside
the lens but not all. I gave the elementary example of a
meniscus lens. A simple box camera lens is a meniscus type
but so are the single cells of lenses like the Zeiss
Convertible Protar, Goerz Dagor, Turner-Reich, and many
other similar lenses. The principal points of the single
elements are such that when one lies generally about the
center of the lens and the other about at the apex of the
convex side although both may displaced toward the convex
side in some lenses. This can be useful because a single
Dagor or Protar cell which has just a bit too long a focal
length for the bellows of a camera when mounted in optimum
way, that is behind the stop and with the convex side facing
the film, may prove usable when turned around. The
difference is not just the effective extension gained by the
thickness of the shutter and lens but because the image is
now coming from somewhere around the center of the lens
rather than its apex. The effect is listed in optical data
as the "telephoto" factor and may be positive or negative so
it accounts for both types.
So, the reversed telephoto lens offers some advantages:
longer distance from lens to film and reduced light fall
off, but has the disadvantages of being more difficult to
correct for some important aberrations. They tend to be more
complex for a given focal length, speed, and level of
performance, than a standard lens but make possible the use
of relatively short focal length lenses on cameras with
limited clearance behind the lens.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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