Help - how to use a convertible lens
Doc
Jean-David Beyer
I do not know that lens, but I do have a Wisner convertable Plasmat set,
and the instructions for that are very clear. (An advantage of the
Wisner set is that there are more than three cells, so you can use
various combinations of two cells to get most of the focal lengths.)
If it is claimed you can get three focal lengths, that means the two
cells you have are of different focal length. Making up the numbers, say
one is 300mm and the other is 400mm. You use one cell or the other to
get a lens of 300 or 400mm (obviously). When doing this, you typically
put the cell behind the shutter (more precisely, behind the aperture
diaphragm). When you use both cells at once, you get an even shorter
focal length (perhaps 210mm) than either of the cells.
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Richard Knoppow
"Doc" <tom....@charter.net> wrote in message
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Tom
Convertible lenses like the T-R or Zeiss Convertible
Protar consist of single cells which are corrected so that
they can be used alone or in combination.
When used in combination the longer focal length lens
should go in front of the iris. Ideally, when used alone,
the single cell should be in back of the stop.
The focal lengths are marked on each cell. On T-R triple
convertibles the combined FL is marked on the front cell.
The speed of the individual cells is around f/12.5.
To get the various focal lengths use either both cells, or
only one cell.
While the corrections are theoretically better with single
cells in back of the stop i.e. on the back of the shutter,
these lenses are slightly retrofocus when used that way and
slightly telephoto when used on the front of the shutter. A
very long FL may run you out of bellows draw if mounted in
the "ideal" position but give you some working space when
mounted on the front.
Now the bad news. The T-R is not a particularly good lens.
Its performance is acceptable as a combined lens but the
individual cells show considerable color fringing. They
really need to be used with a yellow or green filter for
good sharpness. The fringing is visible on the ground glass
toward the margins of the image.
You have one of the older T-R lenses. The company went
through many hands. Late T-Rs were built after a new company
was started in Fairport N.Y., which is near Rochester. Most
of the lenses made in Fairport were made under contract to
the government to fit a spec intended for the Goerz Dagor or
Bausch and Lomb version of the Zeiss Convertible Protar. The
Protar is a very much better lens. Dagors, while patented
and sold as convertibles really are not since the individual
cells are not corrected for coma and must be used at very
small stops (f/45) for good sharpness.
T-R lenses were probably designed by Ernst Gundlach
despite the names of Turner and Reich being on the lens.
They were originally Gundlach's partners. Neither was a lens
designer and the overall design approach is one Gundlach
used in other lenses. Ernst Gundlach designed several lenses
which are essentially rip-offs of lenses which were
protected by patents at the time. The T-R is, essentially, a
Series VII Protar with one element split into two elements
giving a total of five elements in each cell. This requires
four cemented surfaces in each cell. Cemented surfaces must
be individually ground and polished to an exact match with
their mating surfaces. In addition, each element must be
very carefully centered and edge ground so that all elements
will be coaxial and exactly parallel. Its unlikely that very
many T-R lenses were properly centered. Decentration and
especially tilting of elements can be devastating to
performance.
With all this discouraging news the fact is that your
particular lens may be a good one. At least as a combined
lens the performance is pretty good.
The patent for the T-R is USP 539,370 The prescription
given in the patent does not incude the glass dispersion
constants. One can guess at what the glasses might have been
from old glass catalogues but you can't be sure.
I am quite sure this is more than you ever wanted to know.
--
---
Richard Knoppow
Los Angeles, CA, USA
dick...@ix.netcom.com
Jean-David Beyer
> Convertible lenses like the T-R or Zeiss Convertible Protar consist
> of single cells which are corrected so that they can be used alone or
> in combination. When used in combination the longer focal length lens
> should go in front of the iris. Ideally, when used alone, the single
> cell should be in back of the stop.
Richard:
are you sure about this? I have read this both ways; i.e., some say
the longer focal length should go in front of the diaphragm, and some
say it should go behind. Similarly, while most people say a single cell
of a convertable should go behind the diaphragm, a few say the opposite.
Does it depend on the lens design? The reason I ask is because with Ron
Wisner's Convertable Plasmat Set, I always use the longer focal length
behind the diaphragm. I am pretty sure I do this because Ron Wisner told
me to.
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Richard Knoppow
"Jean-David Beyer" <j@d.b> wrote in message
news:3FE08119.70101@d.b...
of getting closer to the sine condition for distant objects.
This is along the lines of semisymmetrical lenses like the
Goerz Dogmar or 203mm, f/7.7 Kodak Ektar which have some of
the power shifted from front to rear to improve their
correction for distant objects. The same arrangement is
found in modern Plasmat lenses like the Schneider Symmar or
Rodenstock Sironar. Both are semisymmetrical, meaning the
layout of the front and back cells are the same but the
powers are a little different. Those with the old
convertible Symmar will find that the rear cell is shorter
in focal length that the front cell. Since the front cell
will not fit on the back of the shutter in most of these
lenses you will have to turn the lens around. In any case,
you can see the difference by just looking through the two
cells.
The correction of a single meniscus lens depends on its
shape and the stop position. Where the lens is optimized for
a stop not at the lens surface the optimum shape is a
meniscus with both surfaces concave to the object and stop.
Sidney Ray's little book shows a Dagor in front of the stop
but its an error, there are some others in the book. The
stop position is important to correction of coma and
spherical aberration.
Since the single cells of convertibles cover only a rather
narrow angle, and since the location of the stop is probably
not optimum, there is often not a big difference in image
quality with position. Since the bellows draw is
significantly shorter for a cell mounted in front of the
stop this is often a practical way of using very ling focal
lenght cells which may be beyond the focusing capacity of
the camera when mounted normally. This principle was often
used in box cameras where mounting the lens in front of the
stop and shutter resulted in a considerably more compact
camera and protected the shutter from dust. This at the
expense of some reduction of image quality. The differnces
may have been lost in other aberrations since very few of
these lenses were achromatized. This lack of color
correction explains the universal use of orthochromatic film
in box cameras. Actually, the image quality of box camera
lenses is quite surprizingly good.
Convertibles vary all over in quality. IMHO the old Zeiss
Convertible Protar is about the best even including the
Symmar although the Symmar is a better lens when complete.
Many standard optics texts will show the variation of coma
and spherical with lens shape for lenses with the stop in
contact with the lens and with the stop spaced away from the
lens. The optimum shapes are quite different.