Ross Xpres lens design?
roland.rashleigh-berry
But I also read somewhere that the lens was independently designed by an
English mathematician. I've also read that the Xpres was demonstrably better
than the Zeiss Tessar right from the beginning which seems to suggest it was
an independent design. Does anyone out there know definitively the history
of this lens that they can share?
ArtKramr
>From: "roland.rashleigh-berry" roland.rash...@ntlworld.com
>Date: 8/12/02 10:55 AM Pacific Daylight Time
>Message-id: <PzS59.2574$Cy3.1...@newsfep1-win.server.ntli.net>
The Ross Expres is totally unrelated to a Tessar. The Ross is a 5 element
design with a single positive front element, an air spaced double negative
central element and a cemented three element rear group. The Gundlach Radar is
a direct copy of the Ross design. Coverage is in the 50 degree range and the
Expres was originaly designed to be a fast lens for aerial phography. It is of
course a British design. Thousands were produced during WW II..
Arthur Kramer
Visit my WW II B-26 website at:
http://www.coastcomp.com/artkramer
Q.G. de Bakker
The Ross Xpress stems from a time in which German manufacturers (including
Zeiss) were trying to design lenses such that they consisted of two halves
that each could be used separately as well. Large lenses used at that time
on large formats were costly, so photographers wanted such "sets". These
attempts resulted in lenses like Rudolph's Double Protar (produced by Zeiss)
and Höegh's Angulon (produced by Schneider). This 'requirement' to satisfy
photographer's frugality however held the development of better lenses back
quite a bit. Apparently there were less such requirements in the UK.
These lenses all were elaborations of Rudolph's (Zeiss) Protar, the first
anastigmatic lens. This was a lens made up out of two groups, each forming a
meniscus, each consisting of a negative and a positive lens cemented
together (achromats). The first cemented pair was made out of "old" glasses,
the seond out of (then) "new" glasses. The use of new glass having a high
index of refraction and low dispersion in this way allowed Rudolph's lens to
have very little astigmaticism.
The two separate groups of each two cemented elements could not each be used
independently. Rudolph solved this by cementing both groups together and
adding yet another such cemented Protar, reversed. Now (very unsurprisingly)
each half of this Double Protar could be used as a separate lens.
Höegh (working for Goerz) simplified this design by replacing two of the
negative lenses made of glass with a medium index of refraction that were
present in each cemented half of the Double Protar by a single negative
lens. Each half of the resulting Dagor now only consisted of three elements:
a double convex positive lens, next a double concave negative lens, and then
a positive meniscus (convex-concave) lens. The whole group again (as in all
the other designs) formed a meniscus.
Other arrangments of these three elements per group are possible. Höegh, for
instance, in his Angulon put the meniscus lens on the outside, followed by
the positive lens and the negative lens.
Voigtländer too changed the order of the three elements in each group,
putting the meniscus in the middle (Collinear).
The meniscus lens in all these double anastigmats was made out of glass with
a low refractive index, and very low dispersion. It was clear that a lens
could be made satisfying all demands at least as well as the other double
anastigmats if this middle meniscus had an even lower refractive index and
dispersion: so in the Ross Xpress it was left out completely, i.e. was
replaced with an "air lens".
More development of the double anastigmat took place (like the Double
Plasmat), but need not concern us here.
So the Ross Xpress was a result of trying to improve the double anastigmat.
It resembles the Tessar in that it too has first a positive lens, than a
negative lens, and finally a cemented group. The difference is that this
cemented group in the Tessar is an achromat, in the Xpress it's Höegh's
version of a cemented anastigmat.
The Zeiss Tessar has a quite different history. It is based on Petzval's
formula and it's consequences for anastigmatism.
Petzval's condition for anastigmatism neither spatial separation nor
thickness of lenses are parameters. While the second bit was made use of in
designing the above mentioned double anastigmats, H. Dennis Taylor
(scientific assistant at Cooke's) used the first bit to design a simple but
excellent lens, the Cooke Triplet.
Petzval says that for a single lens to be free from astigmatism, the
curvature of both it's surfaces should be equal. A planparallel block of
glass satisfies this condition. A planparallel block of glass however has no
refractive power. If we divide this block into three parts, one plan-convex
(positive lens), one double concave (negative lens), the last convex-plan
(positive lens), nothing changes: no refractive power, but no curvature of
field and astigmatisme either. But since spatial separation between these
elements is not a parameter, we can move these three pieces apart and the
resulting compound lens will have refractive power, but still no
astigmatism.
There are other aberrations still left to correct, and one thing that was
done was reverse the two outer, positive elements, lessening spherical
aberration. Using different types of glass helps correct chromatic
aberration, while distortion can be controled by placement of the diaphragm.
Cooke's Triplet was almost completely free from spherical aberration and
coma.
Not very surprising perhaps that such a simple, cheap to make lens stirred
quite some interest when it appeared in 1894. The German manufacturers in
particuklar were quick to adopt the design and tried to improve it even
further. The way to do that was obvious: take each element of the triplet
and try to improve it by replacing it by cemented compound lenses.
Voigtländer's designers Harting and Deser both came up with their versions,
the Heliar and the Heliostigmat; and Leitz's Berek designed the Hektor.
The Tessar is another example of these attempts to improve the Cooke's
Triplet: in it the last positive lens was replaced by a achromatic doublet.
So no, though both lenses are similar, the Xpress is not (!) a copy of the
Tessar. They are each result of a different way taken in trying to design
better lenses.
ArtKramr
>From: "Q.G. de Bakker" q...@worldonline.nl
>Date: 8/12/02 4:24 PM Pacific Daylight Time
>Message-id: <aj9fpb$1m6$1...@reader1.tiscali.nl>
>The Ross Xpress stems from a time in which German manufacturers (including
>Zeiss) were trying to design lenses such that they consisted of two halves
>that each could be used separately
No the Ross express is neither convertable or symmetrical and the halves
cannot be used alone.
>time
>on large formats were costly, so photographers wanted such "sets". These
>attempts resulted in lenses like Rudolph's Double Protar (produced by Zeiss)
>and Höegh's Angulon (produced by Schneider). This 'requirement' to satisfy
>photographer's frugality however
Wrong. It is totally unrelated to Zats Protar sets And the formula is not
remotely the same.
>These lenses all were elaborations of Rudolph's (Zeiss) Protar, the first
>anastigmatic lens. This was a lens made up out of two groups, each forming a
>meniscus, each consisting of a negative and a positive lens cemented
>together (achromats). The first cemented pair was made out of
You are talking about a Protar, not a Ross expres.
fotocord
Duh, try reading what he wrote.... He said it was NOT a convertable design
later in the post (it "stemed" from a time when they DID use convertable
designs). And the description WAS about a protar NOT a Ross..
Or was this post too long to keep your interest long enough to read the
whole thing?
BTW thanx for the informative post Q...
--
Stacey
Mike
through f4.5 and were used in folding cameras, TLR's and Press/view
and Arial cameras. The original design looks to date back to 1913 and
from what information I have found is either a Tessar or Xenar
variety. (non enough information to be sure). Similar designs are the
Grunlach Radar. I think one of the designers was a person called
Stuart.
There were also process Xpres and Wide Angle Xpres versions availble
as well. It
Cannot find much data, but would be interested if someone can provide
more history/information.
Mike
ArtKramr
>Date: 8/12/02 9:24 PM Pacific Daylight Time
>Message-id: <3d588692...@news.sf.sbcglobal.net>
>esign looks to date back to 1913 and
>from what information I have found is either a Tessar or Xenar
>variety. (non enough information
What information is that? The cross sections aren't even close.
Mike
determination, a google search came up with information that mentions
a tessar or maybe a xenar derivative, I am not a lens construction
expert that is why I asked if anyone had any more detailed
information!
So what is close?
Mike
Doing a google search I came up with information that mentioned On 13
roland.rashleigh-berry
news:20020813003009...@mb-bk.aol.com...
> >Subject: Re: Ross Xpres lens design?
> >From: mki...@nospampacbell.net (Mike)
> >Date: 8/12/02 9:24 PM Pacific Daylight Time
> >Message-id: <3d588692...@news.sf.sbcglobal.net>
>
> >esign looks to date back to 1913 and
> >from what information I have found is either a Tessar or Xenar
> >variety. (non enough information
>
> What information is that? The cross sections aren't even close.
Is there somewhere on the Net that we can look at a cross-sectional diagram
of the Xpres?
fotocord
> Art,I did mention that there was not enough information to make a
> determination, a google search came up with information that mentions
> a tessar or maybe a xenar derivative, I am not a lens construction
> expert that is why I asked if anyone had any more detailed
> information!
>
> So what is close?
>
>
I think Q just gave a nice description of it's history if you read the
whole thing..
--
Stacey
Q.G. de Bakker
> Is there somewhere on the Net that we can look at a cross-sectional
diagram
> of the Xpres?
If you find somewhere to put it up, i will send you a jpeg showing the cross
section of the Xpress and some other double anastigmats.
Q.G. de Bakker
> Art,I did mention that there was not enough information to make a
> determination, a google search came up with information that mentions
> a tessar or maybe a xenar derivative, I am not a lens construction
> expert that is why I asked if anyone had any more detailed
> information!
>
> So what is close?
As i wrote earlier, the designs of both Tessar and Xpress look very similar.
Each being a triplet with first a double convex positive lens, an airspace,
then a double concave negative lens, another airspace, and finally a
cemented, compound lens. In the Tessar design this third group is of the
simple achromat design (negative lens and positive lens kitted together). In
the Xpress the third group is Voigtländer's version of Höegh's simplified
version of Rudolph's cemented Protar half of the Double Protar ;-) (negative
lens, meniscus, positive lens, the entire group forming another, thick
(remember Petzval?) meniscus).
And as i also mentioned earlier, the Xpress is not (!) a Tessar derivative.
The Xpress is the result of refining Rudolph's double anastigmatic Double
Protar. The Tessar is one of many lens designs trying to improve on Cooke's
original Triplet.
ArtKramr
>Date: 8/12/02 11:37 PM Pacific Daylight Time
>Message-id: <wK169.63$jb4....@newsfep1-win.server.ntli.net>
I have the cross sections in my copy of Henney & Dudley Handbook of
Photography.And you will also find it in Kingslake.
ArtKramr
>Message-id: <ajas49$gbg$1...@reader1.tiscali.nl>
>
>Mike wrote:
>In
>the Xpress the third group is Voigtländer's version of Höegh's simplified
>version of Rudolph's cemented Protar half of the Double Protar ;-) (negative
>lens, meniscus, positive lens, the entire group forming another, thick
>(remember Petzval?) meniscus).
>
>And as i also mentioned earlier, the Xpress is not (!) a Tessar derivative.
>The Xpress is the result of refining Rudolph's double anastigmatic Double
>Protar. The Tessar is one of many lens designs trying to improve on Cooke's
>original Triplet.
>
Well, the Protar reference escapes me. I just don't see the similiarity in any
manner or form between a Protar and a Ross Expres.And it is my feeling that
bringing on Protars in a Ross Expres discussion may serve to only muddy the
waters. Besides, the covering power of the Xpress is onyl 50 degrees while the
Protars go out ot 70 degrees at full aperture and over 80 degrees at F/32. And
those are the regular Protars, the wide angle (f/18) Protars go to nearly 100
degrees.. Performance unrelated to the Xpress designs.
Q.G. de Bakker
> Well, the Protar reference escapes me. I just don't see the similiarity in
any
> manner or form between a Protar and a Ross Expres.
You should read what i wrote. It's all there. The entire evolution from
Protar to Double Protar, to Dagor, to Collinear and next Xpress.
> And it is my feeling that
> bringing on Protars in a Ross Expres discussion may serve to only muddy
the
> waters.
On the contrary. It shows that the Xpress hails from an entirely different
school of though in lens design than the Tessar. And that was the point of
the original question; knowing whether or not the Xpress is a Tessar copy.
It obviously isn't.
> Besides, the covering power of the Xpress is onyl 50 degrees while the
> Protars go out ot 70 degrees at full aperture and over 80 degrees at F/32.
And
> those are the regular Protars, the wide angle (f/18) Protars go to nearly
100
> degrees.. Performance unrelated to the Xpress designs.
So what's your point?
Whatever it may be, it is my feeling that bringing on covering power in a
Ross Xpress (not Expres... ;-)) origin discussion may only serve to muddy
the waters... ;-)
roland.rashleigh-berry
I see it. Has a cemented triplet at the back rather than the Tessar
doublet - otherwise effectively the same design. And there was me thinking
it was a four element design.
ArtKramr
>Message-id: <ajbg7r$p7l$1...@reader1.tiscali.nl>
>
If we are talking about relating a Ross lens to the Protar design, it isn't the
Xpres it is the Homocentric. This is a symetrical 4 element all air spaced
design that is double convertable into two focal lengths.
While Protars were not airsaced, the Homocentric is a lot closer than the
Expres.
roland.rashleigh-berry
Is it showing what is effectively the same Tessar design but with a cemented
triplet at the rear rather than a cemented doublet? This is what my
Kingslake book is showing.
roland.rashleigh-berry
message news:jIb69.4826$ml.2...@newsfep2-win.server.ntli.net...
Art, could you confirm that the diagram in your copy of Henney and Dudley of
the Xpres shows a Tessar design but with the rear cemented doublet replaced
with a cemented triplet?
Cheers,
Roland
Q.G. de Bakker
> Art, could you confirm that the diagram in your copy of Henney and Dudley
of
> the Xpres shows a Tessar design but with the rear cemented doublet
replaced
> with a cemented triplet?
I can confirm you that it certainly does not. It shows a Voigtländer
Collinear type double anastigmat design, with the meniscus in the first
triplet replaced with an "air lens".
I thought that would be clear by now ;-)
fotocord
Roland probably has you killfiled and Art isn't reading what you wrote.. <G>
--
Stacey
Robert Monaghan
quote:
From Rollei Mailing List;
Date: Wed, 10 Jan 2001
From: Richard Knoppow dick...@ix.netcom.com
Subject: Re: [Rollei] OT Ross Xpres lens
you wrote:
>Does anyone here know the truth about the Ross Xpres lens, is it a four
>element, three groups lens like the Tessar, or a five element three
groups
>design like the Heliar? I got one on my Ensign Selfix 820 camera.
>
>/Patric
There are two lenses of this name.
The Ross Xpres is a five element lens. It is similar to a Tessar but the
rear component consists of three cemented elements rather than two as in
the Tessar.
There is also the Wide Angle Xpres, made for aerial survey work. This lens
is a Plasmat type, similar to many current large-format lenses. The
Plasmat is derived from the Dagor by splitting off the inner positive
element and air spacing it. This results in additional degrees of freedom
for the designer, the practical effect of which is the substantial
reduction of zonal spherical aberration, an inherant problem with Dagor
type lenses.
The W.A. Expres is covered under USP 1,1777,262 and BP 195,519 Hasselkus,
et.al. assigned to Ross. I don't have the USP for the Xpres but the BP is
29,637. I don't know what was attempted or claimed for its design. The
Tessar patent would still have been in effect at the time the Xpres was
patented so it may have been no more than an attempt to get around the
Zeiss patent, as was the Gundlach Radar, another Tessar type with three
elements in the rear component.
Zeiss, and others, also had designs for Tessar types with three cemented
elements in the rear (the Biotessar for example) in an attempt to improve
correction for fast lenses. Evidently other generic types proved more
profitable.
The Heliar has cemented elements at each end, so, while it has five
elements, the arrangement is different than in the Xpres. Hans Harting, of
Voigtlander, derived the Heliar from the Cooke Triplet in an attempt to
improve the correction of the triplet. His original lens was symmetrical
with the negative elements facing outward. This was not a very sucessful
lens so he tried reversing the powers of the outside elements with more
success. This lens was originally called the Dynar, but the name was
abandoned by Voigtlander in favor of Heliar. Later designers abandoned
symmetry and got better corrections. The type has never been very popular.
Other than Voigtlander, who held the original patents, about the only
other lenses of this type made widely were the Dallmeyer Pentac designed
by Lionel Booth and a series of lenses designed by Fred Altman of Kodak.
Altman's lenses include an f/3.5 lens for the Medalist camera, a similar
f/3.7 lens for general use on 2x3 press cameras, and a series of enlarging
lenses sold as Enlarging Ektar. These lenses have a reputation of being
exceptionally sharp.
Altman's design is covered in USP 2,279,384
As a side note, any US patent can be gotten on line from the US Patent and
Trade Mark office site at http://www.uspto.gov They are in the form of
images in Class-4 TIFF files. The best plug-in viewer is "Alternatiff" you
will have to do a web search to find it, its free.
The Wang Imaging viewer built into Windows will read the files off-line
and print them. The Wang program is no longer available free from
Microsoft but its in most editions of Win 95 and 98. Search the Windows
folder for it. The USPTO now has all patents from 1790 on line but older
patents can be searched only by patent number. These things were probably
scanned with a FAX machine so the quality is not always good. That also
accounts for the strange TIFF format, its what is used for FAX.
I don't know if British patents are on line anywhere.
----
Richard Knoppow
Los Angeles,Ca.
dick...@ix.netcom.com
--
* Robert Monaghan POB752182 So. Methodist Univ., Dallas Tx 75275 *
* Third Party 35mm Lenses: http://medfmt.8k.com/third/index.html *
* Medium Format Cameras: http://medfmt.8k.com/mf/index.html *
Don James
<roland.rash...@ntlworld.com> wrote:
>
>Is it showing what is effectively the same Tessar design but with a cemented
>triplet at the rear rather than a cemented doublet? This is what my
>Kingslake book is showing.
>
>
Kingslake does in fact show very similar cross-sections. Not sure why
Art claims they're "not even close", but cites Kingslake as a
reference (Art, are you referring to the WA Xpres?). Kingslake also
claims that the Xpres (pat. 1913) is a Tessar (pat. 1902) derivative,
replacing the rear doublet with a triplet to get around the Zeiss
patent. However, Q's explanation describes two independent paths to a
similar design; unfortunately I don't have the resources to follow his
trail. I also don't want to cross-section my lenses to settle the
similarity argument.
Would anyone care to argue that one or the other is a superior design?
Michael Gudzinowicz
> roland.rashleigh-berry wrote:
>
> > Art, could you confirm that the diagram in your copy of Henney and Dudley of
> > the Xpres shows a Tessar design but with the rear cemented doublet replaced
> > with a cemented triplet?
(Fig. 6.7a), however, in the H&D represenation the curves seem to have a
shorter radii, and the negative power or focal length of the front cell is
more obvious. The rear cell is a cemented triplet.
> I can confirm you that it certainly does not. It shows a Voigtlander
> Collinear type double anastigmat design, with the meniscus in the first
> triplet replaced with an "air lens".
> I thought that would be clear by now ;-)
Henny and Dudley clearly indicate that the design was derived from
the Tessar, with the Xpres using a triplet rear group to provide
additional corrections. The Xpres is listed in the Tessar category, and
the Collinear is in the symmetrical anastigmat category where it belongs.
The apparent arrangement of elements in the rear group of of the Ross
lens may very superficially resemble one cell of the Collinear, however, to
imply that the overall lens design is similar or derivitive is incorrect.
The Collinear relies on symmetry for some corrections (coma, distortion,
lateral chromatic aberration) not afforded by the single cell. Each
cell has a positive focal length or power, so the second symmetrical cell
essentially reverses some of the aberrations introduced by the first. The
focal length of the complete lens is less than that of each cell.
The Ross Xpres relies on a completely different approach, pioneered by the
Cooke triplet and Tessar. The front two element cell has a negative power
or focal length and the rear component is a highly convergent group. The
combination results in a complete lens where the focal length is longer
than that of the positive rear cell alone. The lens is highly asymmetrical
and the optimization approach is very different from symmetrical lenses.
There aren't any "freebee" corrections due to symmetry around the stop.
The history of the lenses is well known from manufacturers, designers and
patents, and is covered in most good books on optical design.
Which references did you rely upon that state that the Xpres was derived
from the symmetrical design approach rather than the triplet anastigmat
approach?
ArtKramr
>Date: 8/13/02 7:08 PM Pacific Daylight Time
>Message-id: <ajce3h$6b2$1...@freenet9.carleton.ca>
You are right of course, it wasn't, as anyone can tell at a single glance. See
my post on the 8" F/2.9 Expres.. A lens I used for many years.
ArtKramr
>Date: 8/13/02 7:19 PM Pacific Daylight Time
>Message-id: <3d59bf6a.52956160@nntp>
It is certainly is a lot closer to a Tessar than a Protar. But isn't quite
correct to simply call it a Tessar which it isn't. The Protar reference just
isn't right.The Protar is a convertable symmetrical design and the 5 element
Xpres is certainly not that..No I was not referring to the WA Expres at all.
See my post on the 8" F/2.9 Xpres.
fotocord
>
> The apparent arrangement of elements in the rear group of of the Ross
> lens may very superficially resemble one cell of the Collinear, however,
> to imply that the overall lens design is similar or derivitive is
> incorrect.
>
Where did he say that? He said the rear element was like it, not the whole
lens design!
--
Stacey
fotocord
>
> Which references did you rely upon that state that the Xpres was derived
> from the symmetrical design approach rather than the triplet anastigmat
> approach?
Missed this one.
Reread Q's original post, he never said this. He said that most lenses
BEFORE this lens were designed as symetrical (so photogs could have several
focal lengths from one lens) but that this one was different, wasn't made
like the convertable lenses of the time but was derived from the cooke
triplet as a non convertable lens. Art someone didn't understand or read
what Q wrote in his original post.
--
Stacey
Lassi Hippeläinen
<...>
> The Ross Xpres relies on a completely different approach, pioneered by the
> Cooke triplet and Tessar. The front two element cell has a negative power
> or focal length and the rear component is a highly convergent group. The
> combination results in a complete lens where the focal length is longer
> than that of the positive rear cell alone.
Oh dear... I didn't realize that Tessar was the first retrofocus design
;-)
-- Lassi
Q.G. de Bakker
> Reread Q's original post, he never said this. He said that most lenses
> BEFORE this lens were designed as symetrical (so photogs could have
several
> focal lengths from one lens) but that this one was different, wasn't made
> like the convertable lenses of the time but was derived from the cooke
> triplet as a non convertable lens. Art someone didn't understand or read
> what Q wrote in his original post.
Yes and no. ;-)
The Xpress was indeed not an attempt to maintain symmetry to ensure that
both halves could be used separately.
But it still is a derivative of those lenses (the double anastigmat family),
and (unlike the Tessar) not of Cooke's Triplet.
Q.G. de Bakker
> > I can confirm you that it certainly does not. It shows a Voigtlander
> > Collinear type double anastigmat design, with the meniscus in the first
> > triplet replaced with an "air lens".
> > I thought that would be clear by now ;-)
>
> That is incorrect and misleading.
>
> Henny and Dudley clearly indicate that the design was derived from
> the Tessar, with the Xpres using a triplet rear group to provide
> additional corrections. The Xpres is listed in the Tessar category, and
> the Collinear is in the symmetrical anastigmat category where it belongs.
Indeed. That's where the Collinear belongs.
The Xpress is very similar to the Tessar, and one could well see why it is
put in that category, but it didn't start life as result of an attempt to
build a better tripler. It's the result of attempts to improve upon the
double anastigmat, attempts that (as mentioned) did not want to maintain
some degree of symmetry at all costs. I suggest that Henny and Dudley, quite
simply, are mistaken. ;-)
> The apparent arrangement of elements in the rear group of of the Ross
> lens may very superficially resemble one cell of the Collinear, however,
to
> imply that the overall lens design is similar or derivitive is incorrect.
Very superficially? It has exactly the same arrangement of optical elements
as Voigtländer's Collinear (A positive, biconvex lens followed by a inwardly
curved meniscus, and a negative, biconcave lens. In both (!) 'halves'),
maintaining the same relation between the refractive index of the elements
that make up both (!) 'halves' too.
> The Collinear relies on symmetry for some corrections (coma, distortion,
> lateral chromatic aberration) not afforded by the single cell. Each
> cell has a positive focal length or power, so the second symmetrical cell
> essentially reverses some of the aberrations introduced by the first.
Same in the Xpress. A lens does not need to be symmetrical to do that, does
it? ;-)
Michael Gudzinowicz
second reversed cell. The Tessar and Xpres derived aren't even close to
that type of design, and represent a totally different approach based on
the triplet. Taylor's papers and patents on the triplet are very clear,
and the Tessar and Xpres patents are modifications to improve that
approach.
The implication that the Xpres is a derivative of the double anastigmat
family based on the superficial resemblence of the rear cell of the
Collinear to the rear group of the Xpres (both 3 element cemented groups,
(-++) is really stretching it, since those groups are far from being
identical. Because a lens uses multiple element cemented groups
does not make it a derivative of the double anastigmat design.
The difference in approach is documented in most books on lens design,
papers, patents and historical treatises.
Do you have a misleading reference or is the statement "it still is a
derivative of those lenses (the double anastigmat family)" purely
speculative, and without support or merit?
ArtKramr
>Message-id: <ajdg1a$r1e$1...@reader1.tiscali.nl>
>
>Michael Gudzinowicz wrote:
>I suggest that Henny and Dudley, quite
>simply, are mistaken. ;-)
And I suggest that they are absolutely accurate. What evidience do you have to
the contrary? Just one glance at the cross section of the Expres indicates
they are corrrect.
ArtKramr
>From: Lassi =?iso-8859-1?Q?Hippel=E4inen?= lahi...@ieee.orgies.invalid
>Date: 8/14/02 12:00 AM Pacific Daylight Time
>Message-id: <3D59FF14...@ieee.orgies.invalid>
>Oh dear... I didn't realize that Tessar was the first retrofocus design
>;-)
>
>-- Lassi
Whaaaaa? I assume that is a joke.
Q.G. de Bakker
> Symmetry is used to correct those aberrations by passage through a
> second reversed cell.
Indeed. But you don't need a symemetrical design to do that. Asymmetrical
designs can do (and do) the same.
> The Tessar and Xpres derived aren't even close to
> that type of design, and represent a totally different approach based on
> the triplet. Taylor's papers and patents on the triplet are very clear,
> and the Tessar and Xpres patents are modifications to improve that
> approach.
Well, no. The fornt half of the Xpress has the same make up as the back
half. The individual elements have different curvatures and thickness, and
one in the front half even has a refractive index of 1 and as close to 0
dispersion as one might get. Yet they are fundamentally the same.
> The implication that the Xpres is a derivative of the double anastigmat
> family based on the superficial resemblence of the rear cell of the
> Collinear to the rear group of the Xpres (both 3 element cemented groups,
> (-++) is really stretching it, since those groups are far from being
> identical.
:-)
As mentioned above, and before, they are basically the same: "a positive,
biconvex lens followed by a inwardly curved meniscus, and a negative,
biconcave lens."
> Because a lens uses multiple element cemented groups
> does not make it a derivative of the double anastigmat design.
That's right. Not persé. But i have mentioned how Höegh simplified Rudolph's
Double Protar, how Voigtländer changed the order of the elements, and how
Ross replaced one of those in the front group by one having a different
refractive index. All the while these lenses were derivatives of Rudolph's
Double Protar, i.e. double anastigmats. The lineage is clear.
> The difference in approach is documented in most books on lens design,
> papers, patents and historical treatises.
True. That is, the difference between double anastigmats and triplets. ;-)
> Do you have a misleading reference or is the statement "it still is a
> derivative of those lenses (the double anastigmat family)" purely
> speculative, and without support or merit?
Neither. ;-)
Lassi Hippeläinen
>
> >Subject: Re: Ross Xpres lens design?
> >From: Lassi =?iso-8859-1?Q?Hippel=E4inen?= lahi...@ieee.orgies.invalid
> >Date: 8/14/02 12:00 AM Pacific Daylight Time
> >Message-id: <3D59FF14...@ieee.orgies.invalid>
>
> >Oh dear... I didn't realize that Tessar was the first retrofocus design
> >;-)
> >
> >-- Lassi
>
> Whaaaaa? I assume that is a joke.
Of course, that's why I put the little grin in there. But if you think
about it:
Michael Gudzinowicz wrote:
<...>
> The Ross Xpres relies on a completely different approach, pioneered by the
> Cooke triplet and Tessar. The front two element cell has a negative power
> or focal length and the rear component is a highly convergent group. The
> combination results in a complete lens where the focal length is longer
> than that of the positive rear cell alone.
1) negative focal length in the front,
2) followed by positive rear,
3) and the total focal length longer than the rear group.
Isn't that exactly a retrofocus design?
Of the lenses listed, the Cooke triplet is the oldest, but also a bit
dubious, because in many versions the center element isn't clearly part
of the front or rear group. But in Tessar there is no doubt; the
aperture/shutter separates front and rear groups unambiguously. So I
called it the first retrofocus lens. But it wasn't originally called so,
because the word was invented half a century later.
-- Lassi
ArtKramr
>From: Lassi =?iso-8859-1?Q?Hippel=E4inen?= lahi...@ieee.orgies.invalid
>Message-id: <3D5A5D0E...@ieee.orgies.invalid>
>Of the lenses listed, the Cooke triplet is the oldest, but also a bit
>dubious, because in many versions the center element isn't clearly part
>of the front or rear group. But in Tessar there is no doubt; the
>aperture/shutter separates front and rear groups unambiguously. So I
>called it the first retrofocus lens. But it wasn't originally called so,
>because the word was invented half a century later.
>
>-- Lassi
>
>
>
>
>
>
Ah the old story. Too little too late. (grin)
Q.G. de Bakker
> And I suggest that they are absolutely accurate. What evidience do you
have to
> the contrary? Just one glance at the cross section of the Expres
indicates
> they are corrrect.
You mean like just one glance at the cross section indicates that i am
correct too? It does, you know... ;-)
brian
> roland.rashleigh-berry wrote:
>
>
> So no, though both lenses are similar, the Xpress is not (!) a copy of the
> Tessar. They are each result of a different way taken in trying to design
> better lenses.
Wrong. The Ross Xpress is virtually a direct Tessar knockoff with a
weak meniscus positive element tossed into the cemented component to
get around Rudolph's patent. The lens uses barium crown glass and a
low-index flint in exactly the same way the Tessar does in order to
achieve a flat field, which is different from the air-spacing
technique used in the Taylor triplet. Willy Merte, who was a Zeiss
designer in the early half of the 20th century and who compiled
several volumes of patent and lens design data had this to say about
the Ross Xpress:
"The f/4.5 Xpress-Lens manufactured by Ross, Ltd, London, comes under
the claims of British patent No. 29637/1913. This lens differs from
the Zeiss Tessar in that the cemented portion consists of three
components instead of two."
Brian Caldwell
www.caldwellphotographic.com
ArtKramr
>Message-id: <ajei69$ikj$1...@reader1.tiscali.nl>
Q.G. de Bakker
> Wrong. The Ross Xpress is virtually a direct Tessar knockoff with a
> weak meniscus positive element tossed into the cemented component to
> get around Rudolph's patent. The lens uses barium crown glass and a
> low-index flint in exactly the same way the Tessar does in order to
> achieve a flat field, which is different from the air-spacing
> technique used in the Taylor triplet. Willy Merte, who was a Zeiss
> designer in the early half of the 20th century and who compiled
> several volumes of patent and lens design data had this to say about
> the Ross Xpress:
>
> "The f/4.5 Xpress-Lens manufactured by Ross, Ltd, London, comes under
> the claims of British patent No. 29637/1913. This lens differs from
> the Zeiss Tessar in that the cemented portion consists of three
> components instead of two."
And how does this make the Xpress a Tessar knock-off?
Q.G. de Bakker
> >You mean like just one glance at the cross section indicates that i am
> >correct too? It does, you know... ;-)
> >
> >
> Afraid not.
Have you even looked at the lenses i mentioned?
fotocord
> But it still is a derivative of those lenses (the double anastigmat
> family), and (unlike the Tessar) not of Cooke's Triplet.
OK
--
Stacey
fotocord
> brian wrote:
>
>>
>> "The f/4.5 Xpress-Lens manufactured by Ross, Ltd, London, comes under
>> the claims of British patent No. 29637/1913. This lens differs from
>> the Zeiss Tessar in that the cemented portion consists of three
>> components instead of two."
>
> And how does this make the Xpress a Tessar knock-off?
Same way a distagon is a knockoff of a ross xpress, they just added some
elements..
--
Stacey
Q.G. de Bakker
> > And how does this make the Xpress a Tessar knock-off?
>
> Same way a distagon is a knockoff of a ross xpress, they just added some
> elements..
Ah! Yes, indeed! ;-))
brian
The Ross Xpress has no performance advantage over a Tessar. IMO its
just different to get around the Zeiss patent that was still in force
at the time.
ArtKramr
superior glass cooling methods resulting in cleaner glass with higher contrast
and lower dispersion within the elements due to relatively artifact free
production methods.
ArtKramr
>Date: 8/15/02 1:55 PM Pacific Daylight Time
>Message-id: <20020815165504...@mb-fr.aol.com>
The above message appeared truncated and should have read as follows:
The later Zeiss Tessar was actually better than the Ross Expres sisnce it was
made at a time when Zeiss had already emulated Nikon's clean room techniques
and
superior glass cooling methods resulting in cleaner glass with higher
contrast
and lower dispersion within the elements due to relatively artifact free
production methods. The Ross lenses not having these advantages were produced
with what was then known as "dirty glass" with lower transmission and contrast
and greater dispersion within the elements due to artifacts within the
elements.