Coverage of Dagor
Sandy King
Does anyone know how spacing affects the, 1) coverage and 2) resolution of
Dagor lenses. From my experience and that of others it is is evident that
existing lenses sold as Dagors do not always perform the same. For
example, I have a 12" f/6.8 Dagor (# 3144481), which will cover (just
barely) 12X20 at infinity. Others report that the 12" Dagor (both Gold
Ring and Golden) vignettes on 12X20 at infinity. And a friend had a 16
1/2" Dagor that would *not* cover the 12X20 format, even though every
specification I have ever seen for the 16 1/2" Dagor indicates that it
should cover this format with a lot of movements on the front available. I
also have a 19" Dagor (Goerz Process Dagor), #223210, which will cover a
circle of approximatley 38" at infinity, providing huge coverage for the
12X20 format.
Have others observed these anomalies in coverage with Dagor lenses? Or
perhaps with other symmetrical lenses like the Protar, Orthostigmat,
Angulon, Gundlach convertible, etc.?
Sandy King
San...@hubcap.clemson.edu
Richard Knoppow
san...@hubcap.clemson.edu (Sandy King) wrote:
>Sandy King
>San...@hubcap.clemson.edu
Hi Sandy, The Goerz catalogues are consistent in giving the image
circle for f:6.8 Dagors as 87deg at f:45. This comes out to an image
circle of about 22 inches for a 12" lens focussed at infinity. The
image circle can be affected by the lens mounting if it vignettes.
Please note that the image circle as specified by the catalogue data
is for the circle of illumination and says nothing about the qulity of
the image. The 1936 Zeiss catalogue (courtesy of Mike Buchmeier)
gives an 18" image circle for a 12" Dagor at "small stops". Comparing
Zeiss with Bausch & Lomb literature on lenses that B&L made under
license from Zeiss suggests that Zeiss was very conservative in rating
lenses.
I can't say for sure how much spacing affects the image circle. It
can affect the quality of the image off axis but the geometry of
lenses and the small variation of the spacing used to optimise lenses
would suggest that it wouldn't by much, I suspect differences in lens
mounting.
If the serial number of the 12" lens is actually 314481 the serial
number information I have for Goerz indicates that it was made around
1911 to 1914, 223310 would be about 1908.
---
Richard Knoppow
Los Angeles, Ca.
dick...@ix.netcom.com
Sandy King
The 1936 Zeiss catalogue (courtesy of Mike Buchmeier)
> gives an 18" image circle for a 12" Dagor at "small stops". Comparing
> Zeiss with Bausch & Lomb literature on lenses that B&L made under
> license from Zeiss suggests that Zeiss was very conservative in rating
> lenses.
>
> If the serial number of the 12" lens is actually 314481 the serial
> number information I have for Goerz indicates that it was made around
> 1911 to 1914, 223310 would be about 1908.
> ---
Richard,
It is curious that the companies (Zeiss, BF&L, Ross) that manufactured the
Dagor lenses would give different (almost always less) coverage figures
than Goerz. I have often wondered if the figures are just conservative, or
due to the characteristics of the lens mount.
BTW, what is your source for the information about the serial number and
date of manufacture of Dagor lenses?
Richard Knoppow
San...@Clemson.edu (Sandy King) wrote:
>In article <54clss$7...@dfw-ixnews9.ix.netcom.com>, dick...@ix.netcom.com wrote:
> The 1936 Zeiss catalogue (courtesy of Mike Buchmeier)
>> gives an 18" image circle for a 12" Dagor at "small stops". Comparing
>> Zeiss with Bausch & Lomb literature on lenses that B&L made under
>> license from Zeiss suggests that Zeiss was very conservative in rating
>> lenses.
>>
>> If the serial number of the 12" lens is actually 314481 the serial
>> number information I have for Goerz indicates that it was made around
>> 1911 to 1914, 223310 would be about 1908.
>> ---
>Richard,
For many years Bausch and Lomb were licensed by Zeiss to make Zeiss
lenses in the US. Comparing the catalogue specs for Tessars one finds
a full plate size difference between the catalogues for the same focal
length and speed. i.e. a 12" f"4.5 series 1C is listed as covering an
8x10 by B&L but only 6-1/2 X 8-1/2 by Zeiss. However Zeiss gives the
coverage for "small stops" as 14-1/2" circle. So Zeiss is giving
plate size for good image quality with the lenses wide open and B&L
for smaller stops where the corners get better. I think this explains
the difference in the ratings by the two companies, Zeiss was just a
little more honest about what the lenses would do.
There is a difference in the circle of illumination and the circle
of good defininition with most lenses, the circle of illumination
being often much larger. The circle of illumination of a Dagor or
similar lens is about as large with the lens wide open as it is
stopped down but the sharpness of the periphery of the image isn't
very good, this area gets lots sharper as you stop down. To contrast,
"dialyte" lenses, like Artars and Dogmars are pretty sharp to the
edges of the image wide open and the coverage (which is limited)
doesn't get larger as the lens is stopped down. Triplets and Tessars
are midway between these. Generally, older wide angle designs ( and
the Dagor really is a wide angle lens) perform best at rather small
stops.
Its considered good optical design practice to vignette the
illumination circle of a lens bewond the point of good performance,
(see _Modern Lens Design_ by Warren Smith or Kingslake's design
books), so thhis _may_ account for smaller coverage angle in some late
versions of old lenses but you would have to derermine is by actual
measurement.
Both my Dagors are very old and both have the advertised coverage,
but I have no good way to measure the quality of the image at the
limit of coverage.
>It is curious that the companies (Zeiss, BF&L, Ross) that manufactured the
>Dagor lenses would give different (almost always less) coverage figures
>than Goerz. I have often wondered if the figures are just conservative, or
>due to the characteristics of the lens mount.
>BTW, what is your source for the information about the serial number and
>date of manufacture of Dagor lenses?
>Sandy
>San...@hubcap.clemson.edu
---
Linda A Whatley
In <sanking-1910...@s22-pm03.clemson.campus.mci.net>
san...@hubcap.clemson.edu (Sandy King) writes:
>
>Does anyone know how spacing affects the, 1) coverage and 2)
resolution of
>Dagor lenses.
Sandy, here are some measurements from an old Goerz Dagor,
10 3/4 in. The lens was made in about 1922 (thanks for the date,
Mike B.), and it's in the original barrel-- so the spacing must
be correct. The "illuminated field" below is the full angular
field covered before vignetting looks like about a stop's worth,
judging by the appearance of the pupil from the focal plane. The
"Focused field" (a new, highly technical term ;-) is where the
resolution falls off to about 5 lines/mm on the bench.
/F stop: 6.8 8 11 16 22 32 45 pinhole
Illum field: 40 50 62 72 78 82 85 88 deg.
Focused field: 66 66 66 68 70 75 80 deg.
I'll let you interpret all this with respect to your lenses and
those of your friends, Sandy. Even though this lens is old, it
probably represents most f/6.8 Dagors. Resolution of a more modern
Dagor for example (thanks to Richard D.), is much the same.
Oh, spacing does affect the coverage. It seems to be chosen to
compromise resolution as you leave the axis, say down to 30 lines/mm or
so. Then the resolution increases back to say, 60 lpmm toward the edge
of the field before it gets worse again and goes off the deep end very
quickly. Other lenses, like the tessar, also make this compromise.
It's the way the astigmatism corrections are done to get the maximum
field. The spacing adjusts this compromise-- along with affecting the
other aberration corrections, I suppose. Don't know about those.
Anyone?
- Larry Whatley
Sandy King
Larry,
Thanks for the information, but in trying to understand what it means I am
missing some very essential concept. How can the illuminated field be less
than the focused field, as your figures show for f/6.8 through f/11?
How can the illuminated field be less than the focused field, as your
chart shows?
>
> /F stop: 6.8 8 11 16 22 32 45 pinhole
>
> Illum field: 40 50 62 72 78 82 85 88 deg.
>
> Focused field: 66 66 66 68 70 75 80 deg.
>
Sandy King
Richard Knoppow
San...@Clemson.edu (Sandy King) wrote:
>Larry,
>Sandy King
Simply that the quality of the image gets worse as you move away
form the center. Some lenses will illuminate a circle that is much
larger than the one with a good image, i.e. the outer parts of the
circle are blurry. Almost all lenses do this when wide open. The
image quality towards the outer parts of the circle will get better as
the lens is stopped down because stopping down reduces certain
aberrations.
Linda A Whatley
(Sandy King) writes:
>
>Larry,
>
>Thanks for the information, but in trying to understand what it means
I am
>missing some very essential concept. How can the illuminated field be
less
>than the focused field, as your figures show for f/6.8 through f/11?
Sandy, the "illuminated field" I mentioned here is the angle where the
lens pupil is vignetted enough to pass only about half the light. I
guessed at this, judging from the shape of the cropped pupil as seen
from this angle (or actually, half this field angle away from the
axis). This is an angle where I would start worrying about the
exposure dropping off; it isn't noticeable on the ground glass, but may
be if one studies the negative or an undodged print, say in the sky.
Past this angle, the illumination keeps dropping off until at the
critical angle of 88 degrees that I saw (Goerz says, of the Dagor, "a
light circle up to an angle of 90 degrees"), no light reaches the film,
regardless of f/stop. At large apertures, this loss of illumination
begins gradually, not too far from the axis; at small apertures it
happens more and more suddenly near 90 degrees (for this lens). Does
all this make sense?
- Larry Whatley
Sandy King
> Sandy, the "illuminated field" I mentioned here is the angle where the
> lens pupil is vignetted enough to pass only about half the light. I
> guessed at this, judging from the shape of the cropped pupil as seen
> from this angle (or actually, half this field angle away from the
> axis).
Larry,
Thanks for this explanation. I had understood the term illuminated field
to apply to the entire image circle of a lenses and had not seen it used
before as your explanation above defines.
I am still interested in the effects of spacing on coverage with Dagors
and other symmetrical type lenses. Would the image circle at f/45 be
larger or smaller with short spacing or long spacing? By this I means the
distance between the front and rear elements!
Sandy King
Linda A Whatley
>...
>I am still interested in the effects of spacing on coverage with Dagors
>and other symmetrical type lenses. Would the image circle at f/45 be
>larger or smaller with short spacing or long spacing? By this I means the
>distance between the front and rear elements!
>
>Sandy King
Sandy, in respacing a 7" Dagor once, it appeared that about a tenth of an
inch greater spacing curves the focal plane about a half inch toward the
lens as measured in the corners of a 5x7 plate. Likewise, spacing the
groups a tenth of an inch closer than optimum curves the focus almost as
much the other way. (This is for the tangential astigmatic focus; the
sagittal focus is affected perhaps a fifth as much.) You can see from
this that the coverage for some reasonable resolution is much affected--
always worse when the spacing is not correct. Using f/45 as you suggest
would make the spacing's effect on the image circle smaller (than using
the wider apertures), but it could still be pretty severe. We probably
shouldn't diddle with the designed spacing and expect an improvement in
any performance parameter.
Oh, I'm sorry I misled you with my own definition of illuminated field.
It's just that at wide apertures it can be much less than we expect
(uniform field lumination). Just take a peek at the lens from the film's
corner. (Does your camera have those ground glass cut-offs at the corners
like mine-- you can see the pupil severly limited by looking through these
peep holes, put there mostly to relieve air pressure when moving the
bellows, by the way.)
- Larry Whatley
Robert Huckabee
The unthinkable has happened. I dropped my Schneider Super Angulon
90 mm f/8. The rear retaining ring has a slight flat spot and the
last third of the glass looks cracked (not separated) on that edge.
I might be able to take a "bore sight" shot without this showing,
but that's not the point of a Super-A, is it? Since the rear group
screws off, can I buy just the rear group? Is repair a good option?
Your suggestions for a good source for repair or replacement are invited.
Maybe I should just shoot myself instead of photography.
TIA,
Bob, weeping in Dallas
Richard Knoppow
exu...@exu.ericsson.se (Robert Huckabee) wrote:
Try contacting Schneider directly and find out from them if anything
can be done. My guess is not but its certainly worth a phone call or
letter.
I don't think they have a web site.
Steve Gundersen
exu...@exu.ericsson.se (Robert Huckabee) wrote:
>The unthinkable has happened. I dropped my Schneider Super Angulon
>90 mm f/8. The rear retaining ring has a slight flat spot and the
>last third of the glass looks cracked (not separated) on that edge.
>I might be able to take a "bore sight" shot without this showing,
>but that's not the point of a Super-A, is it? Since the rear group
>screws off, can I buy just the rear group? Is repair a good option?
>Your suggestions for a good source for repair or replacement are invited.
I had an internal barrel finish problem with a Schneider lens, looked
like a fungus, and didn't act upon it until it began to reached a lens
element. I sent it in to my country's distributor and they sent it off
to the factory in Germany rather than work on it themselves, since it
appeared the glass might need some work. I was never told exactly what
was done, (warranty repair), but it came back looking like new.
Your problem seems similar in the sense that if a repair can be done,
it's probably something that only the factory could do properly. I
haven't the foggiest if that would be cheaper than simply replacing
the rear cell. You'll probably have to check with Schneider and I am
curious about what they have to say. Please let us know.
With sympathy,
Steve
Robert Huckabee
First, I wish to thank all of you who responded with support and advice.
I called Schneider Corp. of America (1-800-645-7239) and talked with Robert
in the repair department. He advised (as did members of this group) that the
shutter should also be checked and aligned.
Cost:
new rear element (group) $ 150
new shutter $ 120
+ S&H and hopefuly no
additional damage
This is less than the price of a good used S-A 90 f/8. I have decided to send
the lens to them for repair.
The rear element has to be ordered from Germany and will take 2-3 weeks.
Additionaly, Schneider USA is moving to a new location next week. The added
confusion can only increase the turn arround time. I may investigate renting a
90 mm for my November shoot.
Kids, don't try this at home.
Regards,
Bob
Robert Gurfinkel
>>The unthinkable has happened. I dropped my Schneider Super Angulon
>>90 mm f/8. The rear retaining ring has a slight flat spot and the
>>last third of the glass looks cracked (not separated) on that edge.
>>I might be able to take a "bore sight" shot without this showing,
>>but that's not the point of a Super-A, is it? Since the rear group
>>screws off, can I buy just the rear group? Is repair a good option?
> Try contacting Schneider directly and find out from them if anything
>can be done. My guess is not but its certainly worth a phone call or
>letter.
> I don't think they have a web site.
Schneider's NY phone number is 516 496-8500
Kerik 717
A word of warning: I had a similar issue with a G-Claron lens. I was
given a "rough estimate" of $200 for the rear element of my lens. After I
sent in the lens and received the official written estimate, the price was
$450!!! Needless to say, I was pissed.
Kerik