[rollei_list] Re: Back-Focus and Retro-Focus - Flipping lens
- From: "John Wild" <JWild@xxxxxxxxxxxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 10 Jul 2008 18:19:11 +0100
Not that I have studied a Pentagon Six or similar in depth but some
things have always stood out when I have seen one. Firstly that the
viewing screen appears smaller than a Rollei/Hasselblad; secondly that
the lens throat seems closer to the film back than a Rollei/Hasselblad
and thirdly that the viewfinder hood seems higher above the optical axis
than on a Rollei/Hasselblad. This may just be that the look/shape of the
camera gives me a false impression. However, I guess that if a mirror is
closer to the rear of the lens, it can be smaller (and thus the rear of
the lens be closer to the film plane) but as the light has to travel the
same distance to focus, the focusing screen must be further away from
the mirror for the image falling upon it to still be in focus. We are, I
presume, only talking a few millimeters in any direction in respect of
the 'standard' lenses. Looking at the cut-away section at
http://www.praktica-users.com/cams/six/pentaconsixmanual.html, there is
what looks like an optical element between the mirror and viewfinder
screen (under the pentaprism in this particular image), which presumably
will correct any focus/image size issues in such a design. This may
explain my impression of a Pentacon Six being thinner, back to front,
but taller than a Rollei/Hasselblad. I also would guess that it is
cheaper and requires less technical knowlege to design and manufacture a
'standard' optical design (than calculate "retrofocus" etc) and then
'squidge' the design of the body so it is taller but thinner. Is it
possible that such design criteria may have been deemed necessary by the
manufacturer due to lack of skilled optical designers and also available
funding during the period in question.
I await, with eagerness, for the 'blast' from those that will shoot my
perceptions down in flames. ;-)
John
-----Original Message-----
From: rollei_list-bounce@xxxxxxxxxxxxx
[mailto:rollei_list-bounce@xxxxxxxxxxxxx] On Behalf Of Eric Goldstein
Sent: 10 July 2008 15:46
To: rollei_list@xxxxxxxxxxxxx
Subject: [rollei_list] Re: Back-Focus and Retro-Focus - Flipping lens
Yes. Early on I was using the term retrofocus for normal lenses which
have a longer than typical back focus, allowing them to be used on
Rollei and Hasselblad SLRs. An example of such a lens is the 7 element
80/2.8 Planar. As you, Marc, Carlos, and I have all pointed out,
strictly speaking, this is not a Retrofocus (Capital R, registered trade
mark) or reverse telephoto lens. But if you search the net, you will see
many many common usages of the words "retrofocus" and Mark Rabiner's
term "retrofocal" for lenses which have a longer-than-typical back focus
distance, allowing clearance for mirrors, electronics, etc, but which
strictly speaking may not be Retrofocus or reverse tele...
So, hopefully at this point, everyone now understands the nomenclature
in use.
Carlo's excellent summery of the discussion still stands:
"The question has not received a satisfactory answer yet, why some MF
SLR cameras use a standard lens like the Planar 2.8/80 seven elements
with formulas to shorten the physical focal length and other like the
Pentacon Six, Kiev 60, etc. use a standard lens with a physical and
optical real focal length like the Biometar 2.8/80.-"
I suspect Carlo himself may have been in the right neighborhood when he
described some of the viewing limitation of the Praktica-like cameras
reported by users, suggesting an abbreviated mirror and also prism
limitations...
Eric Goldstein
--
On 7/9/08, Richard Knoppow <dickburk@xxxxxxxxxxxxx> wrote:
>
> ----- Original Message ----- From: "FRANK DERNIE"
> <frank.dernie@xxxxxxxxxxxxxx>
> To: <rollei_list@xxxxxxxxxxxxx>
> Sent: Wednesday, July 09, 2008 12:05 PM
> Subject: [rollei_list] Re: Back-Focus and Retro-Focus - Flipping lens
>
>
>
> > does this mean that all the Hassy lenses except the Biogon are
> > retrofocus up to the 150mm?
> >
> >
> >
> I doubt if any other than the wide angle ones are. I am not quite
> sure what emmanual is referring to as pupilarly magnification. Here
> are a couple of possibilities: The pupils of a lens refer to the
> entrance and exit pupils. They are the images of the stop as seen from
> the outside of the lens. The size of the entrance pupil is what
> controls the amount of light entering the lens. In general the pupil
> is not the same size as the physical size of the aperture, rather, it
> is magnified or reduced by the glass in front of it. This also affects
> the location of the pupil. When the glass in front of the stop is
> negative the effective size of the aperture is made smaller than the
> physical aperture and appears to be further away, when the glass is
> positive the aperture is made to appear larger and closer. An example
> of a lens with negative power in front of the aperture is the Tessar,
> an example of a lens with positive power in front of the aperture is
> the Dagor. The location of the pupil in a Tessar is somewhere behind
> the stop, in the Dagor it is in front. A reverse telephoto lens,
> because it has a strong negative element in front will always have a
pupil which is smaller than the aperture.
> Now, this refers to the pupil as seen from the optical axis. As you
> know when a lens is viewed at an angle the stop becomes cat's eye
> shaped. As a result the effetive size of the pupil is smaller at an
> angle than it is on the axis. This is one of the contributors to the
> fall off of light as one moves away from the center of the image. The
> shape of the stop also affects the resolution of the lens since all
> lenses are ultimately limited by diffraction. The diffraction limit is
dependant on the size of the stop.
> Since the stop off axis has a different size in the radial (also
> called
> sagittal) direction than in the tangential direction the resolution
> will be different. Since the stop is larger in the tangential
> direction the resolution of lenses is generally greater for tangential
> lines than for radial lines.
> The the variation of the shape of the stop is affected by the lens
> in front of it. The large negative lens in front of a reverse
> telephoto lens tends to cause less reduction of the size of the stop
> at angles than on axis. As a result the fall off of illumination is
> less than for a standard lens.
> Another possibility of the meaning of stop magnification may be a
> reference to the Roosinov type lens. This is a wide angle lens
> composed of two nearly symmetrical groups that are each similar to a
> reverse telephoto lens. These lenses are very commonly used as wide
> angle lenses and have physically large negative elements on both ends.
> Roosinov introduced some coma into the stop image making the stop
become effectively larger off axis.
> The effect is quite visible when you tilt the lens around. The system
> is known as a tilting entrance pupil because, when the light rays are
> traced through the lens, the position of the pupil is always such as
> to be facing them regardless of the angle of arrival. The light fall
> off of Roosinov type lenses has one less exponent than a standard
> lens. A normal lens (as defined by a bunch of criteria) has a fall off
> of approximately cos^4 theta where theta is the half-angle of the
> image, that is the angular distance of the point of interest from the
> optical axis. This is pretty substantial especially for wide angle
> lenses. The Roosinov lens reduces this fall off to
> cos^3 theta, a substantial increase in illumination at the margins of
> the lens. Not all lenses follow these two rules, for instance the fall
> off can be greater than cos^4 theta, the well known Goerz Hypergon is
> an example. It is also possible to design lenses where the
> illumination actually increases with angle. If a lens is made so that
> the geometry of the image is not rectilinear the illumination is also
> affected, for instance so called fish eye lenses (or f-theta lenses)
> have less fall off than standard lenses. But, the rules are pretty
generally applicable to most lenses.
> Its pretty easy to measure the size of the entrance pupil for the
> purpose of calibrating the stop scale and also easy to measure the
> location of the pupil.
> Measuring the stop size requies a point source of light located at
> exactly the focal distance of the lens. The projected image of the
> stop can be measured by placing a translucent screen over the front of
> the lens. Its distance is not critical since the light will be
> collimated. One can get a reasonable measurement by using a pin-hole
> in a card with a pencil flashlight behind it. A plane mirror is placed
> over the front of the lens so that the light is reflected back through
> the lens to the card. The lens is moved back and forth until the image
> of the pinhole is in focus. Of course, it needs to be offset slightly
> in order to see it. When this is done the lens is focused exactly at
> infinity, i.e., the pinhole is exactly one focal length away from the
> second principal point. The mirror is then removed and the size of the
> projected image is measured. This is devided into the focal length to
> get the effective f/stop. The exit pupil can be measured in the same
> way by turning the lens around and repeating the process of focusing
and measuring.
> To find the location of the pupils one needs a camera capable of
> being focused at a very close distance in order that the image be
> large enough to be easily visible. A view camera works well.
> First focus the camera on some convenient part of the lens, say the
> rim of the lens mount. Now, move the entire camera back and forth
> until the image of the iris is in focus. The distance the camera moved
> is the difference in distance between the reference point and the
> pupil. For instance in measuring a Tessar the camera will move forward
> a bit. This distance should be recorded. If the reference point was
> the rim of the lens mount the distance and direction the camera moved
> will be the distance and direction of the pupil from this point.
> Knowing the location of the entrance pupil can be useful in some
> calculations and because it is the correct point of rotation for
> panoramic pictures. Of course the exit pupil can be measured in the
same way by turning the lens around.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>
> ---
> Rollei List
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