question about Schneider-Kreuznach lens

"Matt Clara" wrote in message
. ..

"Richard Knoppow" wrote in
message
ink.net...

"2Capture" wrote in message
oups.com...
intersting.,..never knew that the lens is 2 part and can
change to
magnify more.

Isn't this regarded as one of the sharpest lens'?

Its a very good lens but the convertible feature
required some compromise in design. The later version was
not made to be convertible and has somewhat better
correction. However, many lenses can be used as
"convertibles" provided the cells will form an image.
Symmars, Dagors, Convertible Protars, are all essentially
double meniscus lenses and either side may be used alone.
The image quality is not as good as the combined lens and
coverage is less.
The Zeiss Convertible Protar, and to some degree the
Convertible Symmar, have each cell corrected for coma,
Dagor cells do not. In a combined lens this aberration is
corrected by the symmetry. Coma causes spots of light to
become blured in a tear-drop shape away from the center
of the image. It is reduced by stopping the lens down.
Dagor cells don't get acceptably sharp at the corners
until about f/45. A Convertible Protar or Symmar will be
sharp at somewhat larger stops.
Convertible lenses were very popular in the days when
most LF images were contact printed because they are
economical. Some were sold in sets, for instance, the
Zeiss Convertible Protar (also made by Bausch & Lomb) was
sold in sets of up to four cells of different focal
lengths which could be used individually or in groups for
a variety of focal lengths.


And Cooke has brought back the Series XV with the XVa.
http://tinyurl.com/hq9w3 (goes to Cooke Optics website)

The TT&H Cooke Convertible Series 15 was a unique
design. Each cell has four elements in two air-spaced
groups. There may be some advantage to this construction but
the original lens had excessive flare because of having four
glass-air surfaces in each cell, double the number of the
Zeiss Convertible Protar. Since the new Cooke lens is
multi-coated this is of no concern. I don't have the patent
for the original so I don't know what the design approach
was. Older English patents do not seem to be available on
the web. However, its designer, H.W.Lee, was one of the
great lens designers so I have no doubt its an outstanding
lens.
It would be interesting to know the merits of the
design relative to the Plasmat type which is the basis of
most of the large format camera lenses and a great many
enlarging lenses for all formates today.
The Plasmat is derived from the Dagor by air-spacing the
negative element of each half. This gives the designer the
ability to change the curvature of the two surfaces plus
he/she has the spacing as another variable. Compared to the
Dagor a Plasmat has much less spherical aberration (a
problem wtih Dagors) and is capable of excellent correction
for astigmatism. The application of air-spacing to the
Protar type may have resulted in a similar improvement but
it may not be much compared to a modern Plasmat. BTW,
Plasmats also suffered from excessive flare so the design
was not much exploited until the availability of good lens
coatings post WW-2.


--
---
Richard Knoppow
Los Angeles, CA, USA

On 13 Sep 2006 04:54:04 -0700, "Dan Fromm"
wrote:

Hmm. You have a lens whose maximum aperture is f/5.6 in a shutter
whose aperture scale starts at f/6.8. Your lens cells have been moved
from the shutter they were originally delivered in into another
shutter. You might want to have the shutter's aperture scale replaced
with one that's right for your 210/5.6.

Well, if the aperture scale for the 370mm should start at f12, and
if f12 isthe maximum aperture for the lens, then:

The same should be true when it's in 210 mode, right?
In other words, the maximum aperture is f5.6, and the
aperture settings should _start_ at f5.6. Or something
like that, right?

On Sun, 24 Sep 2006 18:45:26 -0700, wrote:

On 13 Sep 2006 04:54:04 -0700, "Dan Fromm"
wrote:

Hmm. You have a lens whose maximum aperture is f/5.6 in a shutter
whose aperture scale starts at f/6.8. Your lens cells have been moved
from the shutter they were originally delivered in into another
shutter. You might want to have the shutter's aperture scale replaced
with one that's right for your 210/5.6.

Well, if the aperture scale for the 370mm should start at f12, and
if f12 isthe maximum aperture for the lens, then:

The same should be true when it's in 210 mode, right?
In other words, the maximum aperture is f5.6, and the
aperture settings should _start_ at f5.6. Or something
like that, right?


I mean, when I was using this lens as a normal 210mm lens
in class, I did not have problems with getting aperture
settings and etc. right, and got quite nice results, and
I was a beginning photography student who jumped
into this with other intermediate to advanced people.

So this setup can't be quite all that bad......

In article ,
wrote:

On 13 Sep 2006 04:54:04 -0700, "Dan Fromm"
wrote:

Hmm. You have a lens whose maximum aperture is f/5.6 in a shutter
whose aperture scale starts at f/6.8. Your lens cells have been moved
from the shutter they were originally delivered in into another
shutter. You might want to have the shutter's aperture scale replaced
with one that's right for your 210/5.6.

Well, if the aperture scale for the 370mm should start at f12, and
if f12 isthe maximum aperture for the lens, then:

The same should be true when it's in 210 mode, right?
In other words, the maximum aperture is f5.6, and the
aperture settings should _start_ at f5.6. Or something
like that, right?

You'd think: but the configuration of the elements probably
has different light gathering abilities therefore under different FL's
give differing exposures. even when they share the same over all
opening of the aperture.
--
Reality-Is finding that perfect picture
and never looking back.

www.gregblankphoto.com

In article ,
wrote:

On Sun, 24 Sep 2006 18:45:26 -0700, wrote:

On 13 Sep 2006 04:54:04 -0700, "Dan Fromm"
wrote:

Hmm. You have a lens whose maximum aperture is f/5.6 in a shutter
whose aperture scale starts at f/6.8. Your lens cells have been moved
from the shutter they were originally delivered in into another
shutter. You might want to have the shutter's aperture scale replaced
with one that's right for your 210/5.6.

Well, if the aperture scale for the 370mm should start at f12, and
if f12 isthe maximum aperture for the lens, then:

The same should be true when it's in 210 mode, right?
In other words, the maximum aperture is f5.6, and the
aperture settings should _start_ at f5.6. Or something
like that, right?


I mean, when I was using this lens as a normal 210mm lens
in class, I did not have problems with getting aperture
settings and etc. right, and got quite nice results, and
I was a beginning photography student who jumped
into this with other intermediate to advanced people.

So this setup can't be quite all that bad......

Hum? a relative beginner? looking for validation without examples
for proof.
--
Reality-Is finding that perfect picture
and never looking back.

www.gregblankphoto.com

wrote in message
...
hi,

I've got a Schneider-Kreuznach lens, Symmar 1: 5, 6/210,
1: 12/370

on my 4x5 camera. I know it's a standard 210mm lens, but
what
does the 1: 12/370 part mean?

thanks,

tracy

I've just gone over this thread again. I missed the part
about the aperture scale on the shutter starting at f/6.8.
Its not the right scale for this lens so probably the lens
was remounted in this shutter. Its not difficult to
calibrate the f/stops of a lens, I've posted instructions
here before but will do so again. Symmar lenses were
supplied in Compur shutters and were mounted without
adaptors so probably the cell spacing is OK. You can check
this by focusing on a very distant object and seeing how
sharp the image stays at the corners. If the cell spacing is
way off the marginal image will be quite blurry.
The f/stops of a lens are not in general the physical
size of the aperture but rather the size of what is called
the entrance pupil. The entrance pupil is the _image_ of the
aperture as seen from the front of the lens. It is either
magnified or reduced by the front cell, depending on where
the front cell is positive (as in the Symmar) or negative
(as in a Triplet or Tessar). Where there is only a single
lens located behind the aperture stop, as in many
convertible lenses when only one cell is used, the aperture
stop _is_ the entrance pupil and its physical size is the
quantity you want. Usually, the marked focal length is close
enough to the actual focal length to use it for calculating
the stop. Just devide the focal length by the diameter of
the stop.
To measure the size of the entrance pupil you will need a
white card with a small hole in it and a small light source,
a pencil flashlight will do. You also need a flat mirror
that will fit over the front of the lens. While a
first-surface mirror is ideal an ordinary shaving or makeup
mirror will do providing its flat (not magnifying).
The first step is to focus the lens exactly at infinity.
This is done by "autocollimating", the lens acts as its own
collimator. A collimator is a device that emmits light that
is in parallel lines, that is, appears to come from an
infinite distance.
Place the flat mirror over the lens. Place the white card
behind the lens with the flashlight behind the hole. The
mirror will reflect the image of the light back to the card.
Adjust the reflection so that the image is near the hole.
Focus for as sharp an image as you can get. The lens is now
focused exactly at infinity. Now, leave the card and
flashlight in place and remove the mirror. Place a
translucent screen over the lens, ordinary thin writing
paper will do. There will be a round circle of light on the
paper. This is the entrance pupil. Measure its diameter and
devide the focal length of the lens by it. This is the
f/stop. Once you have set this up you can calibrate a new
stop scale for the lens.
For stops using only a single element on the back of the
shutter you can still use this method or simply measure the
size of the iris since, again, the physical size of the stop
and the size of the entrance pupil are the same when there
is no lens in front of the iris.
There are a number of ways to make aperture scales
ranging from simply marking them on tape or paint over the
old plate (turn it around and paint the back for instance),
to having a new one engraved.
You can use your view camera as a sort of optical bench
for making these measurements.
This is a pretty old lens but its worth e-mailing
Schneider to find out if they have an aperture plate stuck
away somewhere. They can only say no.
Schneider has a fair amount of information on
discontinued lenses on their web site so you may be able to
find one which shows the proper cell spacing or, perhaps,
the same e-mail could ask about that. If the new shutter is
the same as the original the spacing will be OK.
I have one of these lenses. Its a very good lens and
performance of the back element by itself is reasonably good
at small stops. BTW, the front element can also be used by
itself. The threads of front and back are different, at
least on my lens, so you can't move the front cell to the
back, which is the ideal position for it. However, the
difference in correction between the two positions is small.
There is an advantage to using the cell in front: the cells
of this type of lens has principle points (where the light
appears to come from) located such that the lens appears to
be slightly retrofocus when in the "normal" position on the
back of the shutter, and slightly telephoto when in front.
Thus, the front position reduces bellows draw by more than
just the physical thickness of the shutter. It makes it
possible to use some very long lenses on cameras which do
not have enough bellows draw when the lens is in the "ideal"
position.


--
---
Richard Knoppow
Los Angeles, CA, USA