Using Dagor as convertible lens - a test
Erik Ryberg
Several people on this newsgroup have mentioned the possiblility of
using the Goerz Dagor as a convertible lens. I tested it out with my
5x7 Deardorff and my 7 inch Dagor lens last weekend.
Removing the front element, as recommended by people on this newsgroup,
resulted in doubling the bellows draw and, it appears, doubling the
focal length. The resulting image is quite good but strangely the
corners aren't that great. The lens must cast a huge image, far greater
than 5x7, but there's no question that the corners suffer distortion.
Still, for contact printing anyway I wouldn't hesitate using the lens
this way for landscapes. The corners aren't THAT bad.
Removing the rear element results in a bellows draw of about ten inches
but surprisingly the image is almost exactly the same! So there is some
kind of telephoto thing going on here. I know it sounds odd, but there
you have it. The image is worse than the one made by removing the front
image, but not much worse. It still makes a very good contact print.
I shot Arista 125 film at f22. Looks like you lose about a stop and a
half speed when you remove either element. I focused at about 150 feet.
Erik Ryberg
Jess4203
I was interested to see your report of the test with the Dagor. It seems you
went about it the right way, stopping down to f22 and using the single element
in the rear. If you are using B+W film, there is one more thing you can do to
improve the image, and that is to use a filter. Any filter should remove some
color fringing and improve the image, but the most often suggested filter is an
orange, which I guess generally increases the contrast and darkens the sky. A
yellow or a red are the other likely possibilities, although even a blue or
green could be used, you are still cutting out part of the spectrum, and if the
color suits your artistic purpose, so be it. The darker the filter, the better
it will help with the color aberrations, but obviously in some situations a
really dark filter would ruin the image. Given that the symmetry of the
complete lens is what give most of the color correction, and given that some
color aberrations increase (I think) further out from the lens axis, this
manuever should produce some improvement.
If you look at Ron Wisner's website re: his plasmat set, there is a corrector
plate available which is supposed to help eliminate the aberrations when using
one cell. It has zero magnification but somehow corrects the single cell back
close to what it was when combined. This leads me to suggest that you try a
very mild closeup diopter in front of the shutter just to see if it would
improve the image. It would change the focal length some, but maybe not too
much. If you can see the image degradation in the corners on the ground glass
or in the aerial image, this would be an easy experiment. You might also try a
skylight filter, too. I am not sure exactly why the corrector plate works, but
I believe it is just a glass plate, so maybe a filter would do it. Maybe one
of the lens mayvens will comment.
I do think the color filter trick will help some.
Good Luck,
Roy
Richard Knoppow
The Dagor was patented and sold as a convertible. It can be used
that way but really is not because the individual cells are not
corrected for coma and must be stopped down a _lot_ for reasonable
sharpness. they were intended for use at around f/45.
Coma is a very ugly tear-drop shaped smearing of the image away from
the corners, it increases with distance from the center. Since it is
appected by the stop stopping down gets rid of a lot of it. When the
lens is used complete the two cells cancel each other's coma so there
is very little even wide open.
The individual cells of the famous Zeiss Convertible Protar _are_
corrected for coma. Although they still must be stopped down they
generally have better performance than a half Dagor and will work at
larger stops. This is also true of the Convertible Symmar.
The "telephoto" effect is due to the location of the principle
planes of a meniscus lens. Both the Dagor and Protar are meniscus
lenses. The principle plane is the place the image _seems_ to be
coming from. If the angles of the light rays striking the film are
traces back they will form a cone with its apex on the principle
plane. This plane is sometimes located outside of the physical lens.
For single meniscus lenses one plane is located about at the apex of
the convex side and the other in space on the same side. The bellows
draw will change considerably depending on which way the lens is
facing. Unfortunately, some of the abberations of a single lens are
dependant on where the stop is in relation to the lens. For most
lenses the performance is best when the stop is on the subject side.
This position results in the concave side of the lens facing the film
and requires the longer bellows draw.
The difference in performance of a convetible may not be much
different when its used in the "wrong" position since the location of
the stop is probably not optimum anyway. There are times when the
shorter bellows draw is more important than any small quality loss of
using the single element on the front instead of the back.
---
Richard Knoppow
Los Angeles, Ca.
dick...@ix.netcom.com
Larry Whatley
resolution of the aerial image at the film plane. (hope the table format
makes it to the post) Erik, you can scale these results to your 7" Dagor
(that's my own favorite lens by the way... I have #392662) Hi, Richard;
it's been a while. - Larry Whatley
In. Off Axis: 0 1 2 3 4 5 6 7
lpmm res, f/22: 60 50 50 40 35 40 50 12
Now, using only the rear element (fl = 18", true f/32 reads f/17). True
f/32 was the best aperture in this test.
In. Off Axis: 0 1 2 3 4 5 6 7
lpmm res, f/32: 45 40 30 15 10 7 6 4
Erik Ryberg <ryb...@seanet.com> wrote in message
news:3A566C8E...@seanet.com...
Pam Niedermayer
center to film plane center?
And how in the world did you do this test? Amazing.
Pam
Larry Whatley wrote:
>
> Here's some bench data on a good 10 3/4" Dagor, #396887. This is the visual
> resolution of the aerial image at the film plane. (hope the table format
> makes it to the post) Erik, you can scale these results to your 7" Dagor
> (that's my own favorite lens by the way... I have #392662) Hi, Richard;
> it's been a while. - Larry Whatley
>
> In. Off Axis: 0 1 2 3 4 5 6 7
> lpmm res, f/22: 60 50 50 40 35 40 50 12
>
> Now, using only the rear element (fl = 18", true f/32 reads f/17). True
> f/32 was the best aperture in this test.
>
> In. Off Axis: 0 1 2 3 4 5 6 7
> lpmm res, f/32: 45 40 30 15 10 7 6 4
>...
--
Pamela G. Niedermayer
Pinehill Softworks Inc.
600 W. 28th St., Suite 103
Austin, TX 78705
512-236-1677
http://www.pinehill.com
Larry Whatley
the film, all squared up, as you say. The measurements were done in my
basement where the lens can be placed 50 or so focal lengths from a
resolution pattern. The lens is on a fixture that includes an eyepiece that
can be moved away from the axis, looking at the image at the steps
mentioned. There are a lot of caveats, but I believe the data to be true.
The main concern of this is that your eye may not see the same resolution as
film would capture (and contrast is important but isn't measured). But the
limiting resolution values are pretty close, I believe, and much more
accessible than Modulation Transfer Function measurements. This is a very
interesting experiment, since you can see how the abberations, especially
astigmatism, behave across the film plane.
I have data on other lenses, by the way, most of which I posted here a few
years, ago. If anyone needs this sort of stuff on old tessar, artar, or
rapid rectilinear lenses (or some enlarging lenses), I can share it again.
Most of the data I have is on Dagors, though. I would also trade favors if
anyone needs a lens cell spacing determined or minor polishing, cementing
done.
Cheers,
Larry
Pam Niedermayer <pam_...@cape.com> wrote in message
news:3A587FD0...@cape.com...
Pam Niedermayer
that each light ray (for lack of a better term) coming through the
lens would have a different axis; so to measure you'd have to change
your angle of view all over the place to measure the rays coming in
from the periphery of the lens.
Pam
Sandy King
<lind...@ix.netcom> wrote:
> I have data on other lenses, by the way, most of which I posted here a few
> years, ago. If anyone needs this sort of stuff on old tessar, artar, or
> rapid rectilinear lenses (or some enlarging lenses), I can share it again.
> Most of the data I have is on Dagors, though. I would also trade favors if
> anyone needs a lens cell spacing determined or minor polishing, cementing
> done.
>
> Cheers,
> Larry
Larry,
I would sure like to have the result of your tests of other lenses, but
particulary of other Dagors.
Also, if you have any cell spacing information on 14" Dagors I would
appreciate having it. I have two Dagors of this size (one in an Alphax #4,
the other in an Ilex #4), and the spacing very different, which leads me to
conclude that one or both must be off.
Regards,
Sandy King
Richard Knoppow
>Here's some bench data on a good 10 3/4" Dagor, #396887. This is the visual
>resolution of the aerial image at the film plane. (hope the table format
>makes it to the post) Erik, you can scale these results to your 7" Dagor
>(that's my own favorite lens by the way... I have #392662) Hi, Richard;
>it's been a while. - Larry Whatley
>
>In. Off Axis: 0 1 2 3 4 5 6 7
>lpmm res, f/22: 60 50 50 40 35 40 50 12
>
>Now, using only the rear element (fl = 18", true f/32 reads f/17). True
>f/32 was the best aperture in this test.
>
>In. Off Axis: 0 1 2 3 4 5 6 7
>lpmm res, f/32: 45 40 30 15 10 7 6 4
>
>
Hi, Larry, yes it has. I see you have a new e-mail address.
Greetings to the mysterious "Linda" too.
The above numbers are interesting. The increase in resolution of the
complete lens beyond 4 inches suggests anomolous resolution due to the
combining of lines seen visually. Usually this is accompianied by the
wrong number of lines or reversal of dark and light, but can exist
without either. Have you any idea if this is what happened here?
The 87 degree spec for a Dagor is at f/45. At f/16 its spec'd at 70
deg, but of course, there is no definition of what sort of standard
was applied for quality. Assuming the 6" distance is a legit
measurment its equivalent to about a 60deg image circle, a little
smaller than expected. I have no explanation for this.
The rapid fall off for the single cell is not surprizing, there is a
ton of coma.
60 lp/mm on axis is very good. This lens dates from sometime in the
early to mid 1920's (its in a hole in my serial number list) as should
be noted by those who think old lenses incapable of excellent
performance.
>Erik Ryberg <ryb...@seanet.com> wrote in message
>news:3A566C8E...@seanet.com...
>> Hello,
>>
>> Several people on this newsgroup have mentioned the possiblility of
>> using the Goerz Dagor as a convertible lens. I tested it out with my
>> 5x7 Deardorff and my 7 inch Dagor lens last weekend.
>>
>> Removing the front element, as recommended by people on this newsgroup,
>> resulted in doubling the bellows draw and, it appears, doubling the
>> focal length. The resulting image is quite good but strangely the
>> corners aren't that great. The lens must cast a huge image, far greater
>> than 5x7, but there's no question that the corners suffer distortion.
>> Still, for contact printing anyway I wouldn't hesitate using the lens
>> this way for landscapes. The corners aren't THAT bad.
>>
>> Removing the rear element results in a bellows draw of about ten inches
>> but surprisingly the image is almost exactly the same! So there is some
>> kind of telephoto thing going on here. I know it sounds odd, but there
>> you have it. The image is worse than the one made by removing the front
>> image, but not much worse. It still makes a very good contact print.
>>
>> I shot Arista 125 film at f22. Looks like you lose about a stop and a
>> half speed when you remove either element. I focused at about 150 feet.
>>
>> Erik Ryberg
>>
>
>
---
Richard Knoppow
>Reason I asked is that it seems there would be more than one axis,
>that each light ray (for lack of a better term) coming through the
>lens would have a different axis; so to measure you'd have to change
>your angle of view all over the place to measure the rays coming in
>from the periphery of the lens.
>
>Pam
>
Pam, what Larry is using is the "half angle" but stated in linear
distance. This is OK where the focal length of the lens is stated. It
is the linear distance (or angular distance) from the optical axis of
the lens. This is independant of the angle the light rays are taking
through the lens. It is a line drawn through the apexes coaxial to the
periphery. While image circles are typically stated as the angular
diameter of the circle optical performance is stated as related to the
half-angle, since the field in symmetrical in rotation (for lenses
made with spherical surfaces).
The half-angles can be calculated from:
invertan [ X/ (FL/2)] where X is the linear distance from the optical
axis to the point of interest. Obviously twice this is the angle of
the image circle.
---
Pam Niedermayer
Pam
Richard Knoppow wrote:
>
> Pam Niedermayer <pam_...@cape.com> wrote:
>
> >Reason I asked is that it seems there would be more than one axis,
> >that each light ray (for lack of a better term) coming through the
> >lens would have a different axis; so to measure you'd have to change
> >your angle of view all over the place to measure the rays coming in
> >from the periphery of the lens.
> >
> >Pam
> >
> Pam, what Larry is using is the "half angle" but stated in linear
> distance. This is OK where the focal length of the lens is stated. It
> is the linear distance (or angular distance) from the optical axis of
> the lens. This is independant of the angle the light rays are taking
> through the lens. It is a line drawn through the apexes coaxial to the
> periphery. While image circles are typically stated as the angular
> diameter of the circle optical performance is stated as related to the
> half-angle, since the field in symmetrical in rotation (for lenses
> made with spherical surfaces).
> The half-angles can be calculated from:
> invertan [ X/ (FL/2)] where X is the linear distance from the optical
> axis to the point of interest. Obviously twice this is the angle of
> the image circle.
--
Larry Whatley
some of it piecemeal. I'll mention exactly which lenses I've tested if I
get a chance, in case one is of particular interest.
On spacing, the several dependable Dagors I have are spaced at between .0265
and .0285 the focal length. My10 3/4" lens, for example, was spaced at
.0285 x FL in the original barrell. I've had two other Dagors in
non-original shutters that were not spaced correctly. You know, nowadays,
we don't know exactly how to space these. I mean, it's a little subjective,
and we don't know what performance was originally thought most important.
Grimes has some of the best thoughts on this, but... anyone? I'd be
interested in the spacing values of your two 14" lenses, Sandy.
Sandy King <san...@hubcap.clemson.edu> wrote in message
news:sanking-ya0240800...@news1.lig.bellsouth.net...
Larry Whatley
resolution near the field edge. It turns out that the astigmatism
correction in lenses such as the artar, tessar, and dagor invariably
corrects for field curvature near the edge to get the widest possible field.
The tangential astigmatic focal surface usually bends toward the lens as the
field angle increases, then away, then back toward the lens very sharply.
It frequently comes back near the focal plane at about 28 to 30 degrees
(half angle, dagors), and not coincidentally (!), the radial astigmatic
surface crosses it about there too. So it's not unusual to get resolution
nearly equal the center resolution at a ring near the field edge. I know
what you mean about the resolution pattern aliasing to appear better than it
is, but that's not the case here.
Goerz's marketing claims were not very modest, to be polite. I've never
seen any resolution close to 70 degrees at f/16, but I stop looking at 5 or
10 lpmm. The resolution I see here (again, caveats of the aerial image and
object distance of only 50 - 100 focal lengths) is invariably about 10 lpmm
between 32 and 33 degrees off axis; about 36 degrees at f/32, and up to 40
at f/45. I think Goerz's 87 degree (full field) claim must have been just
before the lights go out ;-) Thanks for your observations and history,
Richard-- as always, a pleasure. (Oh... "linda_aw" is my wife. I've
retired... this is our home account)
And oh, a resolution pattern doesn't identify coma well, but lateral color
sure shows up (rear cell used alone).
- Larry Whatley
Richard Knoppow <dick...@ix.netcom.com> wrote in message
news:3a592303....@news.mindspring.com...
Ted Harris
or asking about using modern lenses as convertibles.
I can tell you from experience that at least the Symmar-S MC works fine as a
convertible. last summer I got dragooned into shooting a wedding at the last
minute. To make a long story short I ended up shooting with the front cell of
the 240 only. Chromes were great. it was an accident but proved the point.
Cheers,
Ted
Ted Harris
Resource Strategy
Henniker, New Hampshire
Richard Knoppow
>Sandy, I don't have the other data typed up, but would be happy to share
>some of it piecemeal. I'll mention exactly which lenses I've tested if I
>get a chance, in case one is of particular interest.
>
>On spacing, the several dependable Dagors I have are spaced at between .0265
>and .0285 the focal length. My10 3/4" lens, for example, was spaced at
>.0285 x FL in the original barrell. I've had two other Dagors in
>non-original shutters that were not spaced correctly. You know, nowadays,
>we don't know exactly how to space these. I mean, it's a little subjective,
>and we don't know what performance was originally thought most important.
>Grimes has some of the best thoughts on this, but... anyone? I'd be
>interested in the spacing values of your two 14" lenses, Sandy.
>
>
Spacing measured from what part of the lens?
Larry Whatley
cells, to be the same as the shutter surfaces. I don't know if that's the
usual way of talking about this or not. It certainly leaves the
construction of the cells as a spoiler. - Larry
Richard Knoppow <dick...@ix.netcom.com> wrote in message
news:3a5a67da...@news.mindspring.com...
Richard Knoppow
>Oh... the spacing convention I use is from the mounting surfaces of the
>cells, to be the same as the shutter surfaces. I don't know if that's the
>usual way of talking about this or not. It certainly leaves the
>construction of the cells as a spoiler. - Larry
>
It would be useful to know the actual length of the air space. You
can measure it by using a depth gauge and and taking the diaphragm as
the reference point, and measuring from each side. The diaphragm blade
thickness is small enough to ignore although it could be measured and
subtracted. You could also measure from the surface of the shutter
body or barrel body and subtract the length of the shutter or barrel.
Lens thickness and spacing is measured at the center line of the
lenses.
The air space should scale with focal length. The diaphragm should
also be in the center of the air space although it isn't in one of my
Dagors, and its shutter is certainly the original.
The spacing is important for Dagors. I no longer have the notes for
the two I have but remember that they are slightly different. I'll
measure again and post what I find. Both are in original shutters. One
dates from the mid 1920's the other from the late 1930's.
The older one is in a Bausch & Lonb Compound which is broken. I've
never seen another, even a broken one, so its not very useful.
Richard Knoppow
>Oh... the spacing convention I use is from the mounting surfaces of the
>cells, to be the same as the shutter surfaces. I don't know if that's the
>usual way of talking about this or not. It certainly leaves the
>construction of the cells as a spoiler. - Larry
>
I measured two Dagors and got rather different numbers.
The spacing in the patent is approximately FL/22, but this is an f/8
lens, not the same design as any of the commercial Dagors.
Lens 1, is a 12" Dagor dating from the mid thirties, its in an Ilex
shutter. The distance from the iris the the center of the rear element
at the center is .284 on one side and .306 on the other. The thickness
of the diaphragm blades was not corrected for so the total distance is
a bit less than .590. This is about FL/14
Lens 2, is a 8.25" FL dating from the mid 1920's. Its in a Bausch &
Lomb Compound. The iris is not centered in the air space. Distances
are .287 and .150 Total .437. This is FL/19.
Both of these are the original shutters.
It would be interesting to get spacing of other Dagors.
Although the Dagor design may have been changed to take advantage of
new glasses there is no way to know other than to make radius
measurements of the surfaces. I may try this with the depth gauge but
I don't think I can measure accurately enough. I seriously doubt that
any change was made in the f/6.8 Dagor between its introduction and
the purchase of Goerz American by Schneider, but that is a guess and
no more.
Richard Knoppow
>"Larry Whatley" <lind...@ix.netcom> wrote:
>>
> I measured two Dagors and got rather different numbers.
>The spacing in the patent is approximately FL/22, but this is an f/8
>lens, not the same design as any of the commercial Dagors.
>Lens 1, is a 12" Dagor dating from the mid thirties, its in an Ilex
>shutter. The distance from the iris the the center of the rear element
>at the center is .284 on one side and .306 on the other. The thickness
>of the diaphragm blades was not corrected for so the total distance is
>a bit less than .590. This is about FL/14
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Copied the wrong number:-( obviously its about FL/20
> Lens 2, is a 8.25" FL dating from the mid 1920's. Its in a Bausch &
>Lomb Compound. The iris is not centered in the air space. Distances
>are .287 and .150 Total .437. This is FL/19.
It would still be useful to get spacings from more lenses. The two
f/6.8 Dagors are close enough to FL/20 to suggest this is the right
value. Even the value from the patent data, for a similar, but not
identical, lens is near to this.
drd...@my-deja.com
I can't remember if it was in a note or a conversation with Larry, but
I remember talking about cell spacing and the possibility that even
slight changes in the refractive indexes of the glasses used would be
enough to cause the designers to change cell spacing within the
formula. I've got three distinct generations of 9 1/2"-ers available,
and they sure look different. (Well... they all have six pieces of
glass in two groups and are symmetrical, but you know what I mean.)
Also, remember that there was a period, including the "Berlin" Dagors,
when some of them "lacked quality control". A few years ago I picked
up a "modern" 8 1/4 Dagor cheap because the owner said it wasn't sharp
in the edges. I found out that whoever mounted it in a new lens board
used thick wood, and the lens wasn't seated properly. As soon as I put
it on a new lens board and got the spacing right, the little sucker
sharpened right up. My point is to agree with the earlier notes ---
that there are lots of dagors out there set with the wrong cell spacing.
Sent via Deja.com
http://www.deja.com/
Sandy King
I have been trying to figure out a way to measure accurately the cell
spacing on my Dagors but am not satisified yet with the results. When I do
I will post the information here. What does appear clear is that cell
spacing plays an important role in coverage. The closer the cell spacing
the wider the coverage.
As a user of ultra-large format equipment this is one of the most important
considerations for me in looking at a Dagor because I am trying to get the
widest coverage possible for the focal length. This is in fact one of the
major reasons Dagors are so popular with ultra-large format users,
especially in focal lengths over about 8 1/2". The 240mm Dagor,with
optimized cell spacing, will cover 11X14 and 7X17, while the 12" Dagor is
capable of covering 12X20. Unfortunatley cell spacing is not always
optimized and I have even seen such anomalies as a 16 1/2" Dagor that
vignetted on 12X20! And one of my 14" Dagors just barely covers, when in
fact it should cover with up to 2" of movement.
Larry pointed out earlier that Goerz was very optimistic in its coversge
claims, and I certainly agree. I have done some testing of the Dagor with a
resolution chart and resolution with the 12" Dagor at the corners of the
12X20 format (which takes us to about 87-88 degrees) is 5 lpm or less.
Certainly no modern manufacturer of lenses would claim coverage at 5 lpm!
Which leads me to another observation. In several previous messages I have
indicated that I use a 360mm Symmar with my 12X20, where at infinity it not
only covers the format but gives me almost 2" of movement before
vignetting. This means that the actual circle of illuminatin has to be
arond 80 degrees or more,and the performance at the extreme corners appears
to be about 10 lpm, better than the Dagor. Several have written me
expressing skepticism about this because the coverage of the Symmar was
never stated to be much over 71 degrees. Obviously the standards for
determining coverage of the Dagor and the Symmar were quite different and
in the former case it appears to have been largely synonymous with the
circle of illumination.
One final note. I also have one of the earliest plasmat lenses, a Rudolph
Hugo Meyer Doppel Plasmat with combined cells of 300mm, and it too has a
circle of illumination of well over 80 degrees. The point I would make here
is that the actual useful coverage (if we contact print) of many of the
plasmat lenses is in fact very close to that of the Dagors, and if cell
spacing is off on the Dagors, it may in fact be greater. How else could it
be that my 360mm Symmar has about 2" more actual coverage than one of my
14" Dagors?
Sandy King
Richard Knoppow
The "Berlin" Dagors of Burke & James are sort of a special case
since they were rather casually assembled out of surplus parts.
Index of refraction might cause an overall change in focal length
depending which elements it happened to and whether it was compensated
for in manufacture. Certainly the focal length of commercial lenses
varies one to the other.
The thickness of an air space is measured at the center line of the
lensn not the edges, for reasons which will be obvious with a little
thought. I find I can get reasonably accurate measurements using a
depth gauge and using the closed down diaphragm as a reference point.
The distance from the diaphragm to the apex of each cell is measured
from the other side. To be completely accurate the thickness of the
diaphragm leaves should be measured and twice this thickness
subtracted from the total measured distance. My guage is too large to
measure the thickness of the iris leaves but I think they are thin
enough to be negligible.
To make the measurement correctly the actual focal length of the
lens should be measured. While approximate FL can be easily measured
without an optical bench, the precise FL is not so easy because of the
precision required.
Nonetheless, it would be interesting to learn the air space
thickness of several lenses, even if only the nominal FL of the lenses
is known.
It would also be interesting to know how often the diaphragm is
actually centered in the air space, as it should be. Neither of my
Dagors is centered although the error in the later one is small.
I've posted the data for my two lenses earlier in this thread.
Sandy King
wrote:
> drd...@my-deja.com wrote:
>
> Nonetheless, it would be interesting to learn the air space
> thickness of several lenses, even if only the nominal FL of the lenses
> is known.
> It would also be interesting to know how often the diaphragm is
> actually centered in the air space, as it should be. Neither of my
> Dagors is centered although the error in the later one is small.
> I've posted the data for my two lenses earlier in this thread.
> ---
> Richard Knoppow
> Los Angeles, Ca.
> dick...@ix.netcom.com
Following is the spacing information on the 6 Dagor lenses I curently own,
with some comments about their characteristics. First, some general
comments.
1. I measured spacing as Richard suggested, from the surface of the lens to
the diaphragm of the lens. Both front and rear separations were taken and
the two averaged for the final calculations. Given the measuring tools and
system employed errors of 5-10% are possible, or expected.
2.With every lens measured spacing on the rear was less than front spacing,
averaging about 25% more for front spacing. In other words, none of thr
lenses were centered exactly around the diaphragm..
3. Comments on coverage are based on stopping down to f/62 and represent
the circle of illumination used for contact printing. Resolution at the
extreme corners with the formats cited 5 lpm or less.
4. Although seversl of the lenses in question may have been originally
mounted by Goerz Am. Op.Co. in their shutters the only lens about which I
have full confidence is the 240mm C. P. Goerz Berlin Dopp Anaastigmat Dagor
which was in an original barrel and mounted for me in shutter by S. K.
Grimes.
5. Calculations were based on the nominal focal length of all lenses.
1)
14² Dagor, Serial # 769439, in Ilex #4
C. P. Goerz Am. Opt. Co.
Spacing 0.0332 X FL
Coated lens,very good performer.
Coverage fair. Barely covers 12X20, vignettes unless absolutely centered on
film axis..
2)
14² Dagor, Serial # 398678, in Alphax #4
No name on ring but presume Goerz Am.
Spacing 0.0278 X FL
Uncoated lens but has bloom, good performer but not equal to #769439.
Coverage very good. Covers 12X20 with 2-3² of movements.
3)
8 1/4 Dagor, Serial # 758560, in Compur #2
C. P. Goerz Am. Opt. Co.
Spacing 0.0238 X FL
Uncoated lens. Recently acquired and not familiar with characteristics.
4)
19² Process Dagor, Serial # 223210, in Betax #5
No name on ring but presume Goerz Am.
Spacing 0.0260 X FL
After coated lens and the front surface was repolished. Good performer.
Coverage excellent. Covers 20X24 with considerable movements.
5)
12² Dagor, Serial # 314481, in Alphax #4
No name on ring but presume Goerz. Am.
Spacing 0.0257 X FL
Uncoated lens. Very good performer.
Coverage excellent. Covers 12X20 plus a tad.
6)
240cm Dopp. Anastigmat Dagor, Serial # 354816 , in Ilex #3 (remounted
from original barrel mount by S. K. Grimes)
C. P. Goerz Berlin
Spacing 0.0263 X FL
Uncoated lens. Good performer.
Coverage excellent. Covers 11X14 and 7X17 plus a tad.
Richard Knoppow
>In article <3a6039c1...@news.mindspring.com>, dick...@ix.netcom.com
>wrote:
>
>> drd...@my-deja.com wrote:
>>
>
>
>> Nonetheless, it would be interesting to learn the air space
>> thickness of several lenses, even if only the nominal FL of the lenses
>> is known.
>> It would also be interesting to know how often the diaphragm is
>> actually centered in the air space, as it should be. Neither of my
>> Dagors is centered although the error in the later one is small.
>> I've posted the data for my two lenses earlier in this thread.
>> ---
>> Richard Knoppow
>> Los Angeles, Ca.
>> dick...@ix.netcom.com
>
>
>Following is the spacing information on the 6 Dagor lenses I curently own,
>with some comments about their characteristics. First, some general
>comments.
>
>1. I measured spacing as Richard suggested, from the surface of the lens to
>the diaphragm of the lens. Both front and rear separations were taken and
>the two averaged for the final calculations.
^^^^^^^^^^^^^^ Do you actually mean avaraged? I think this is a
typo.
Interesting. My two Dagors, both U.S. made in original shutters
measure around .05 x FL or FL/20 This is close to the value in the
Dagor patent, although that is for an f/8 version, not the f/6.8
What I measure is the depth from the iris to the furthest part of the
glass (opposite of apex, can't think of the word). A measurement is
made from either side and the two added. Ideally twice the thickness
of the diaphragm blades should be subtracted but I think the error is
probably negligible. The diaphragm gives a fixed reference point for
both depth and center of the lens.
It would also work to measure the depth by using a reference plate
layed over the end of the shutter. You would have to know the
thickness of the reference plate and the thickness of the shutter from
the reference surfaces. The distance of the glass could then be
measured independantly of the diaphragm, although it still makes a
good locator for the center of the lens.
The problem with the diaphragm blades is that they are flexible
making it touchy to make a good depth measurement. However, accuate
measurement can be made with a little care and delicate touch.
Please re-check one of the above lenses. We are in disagreement by a
factor of two. Perhaps its real but I suspect we are doing something
different in measuring.
Sandy King
wrote:
> Please re-check one of the above lenses. We are in disagreement by a
> factor of two. Perhaps its real but I suspect we are doing something
> different in measuring.
> ---
> Richard Knoppow
> Los Angeles, Ca.
> dick...@ix.netcom.com
Our measurments are very similar because my figures represent only 0.5X of
the average of the total front and rear spacing. In other words all my
figures would need to be multiplied by 2X to equate to your system of
measurement. I measured both distances, but took half of the two distances
as the figure stated.
My reationale for providing the 0.5X figures, all averages of combined
front and rear spacing, results from the fact that my initial *actual*
measurements of cell spacing all approximated 0.05 - 0.06 X FL. At that
point I *assumed* that Larry's figures (0.0265 - 0.0285 X FL) were based on
0.5X of total cell spacing, or to only one of the cells. Sorry for the
confusion.
Sandy King