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Double Gauss Designs
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PostPosted: Tue Jan 07, 2014 3:56 pm    Post subject: Reply with quote
spoilerhead wrote:

compare to:
Zeiss Biogon 21mm ZM


A useful comparison - the lens is Distagon (retrofocus) by design if not by name.

Quote:
the new Zeiss Otus 55 1.4 is also similar:


Described explicitly - by Zeiss - as a Distagon (retrofocus).


PostPosted: Tue Jan 07, 2014 4:03 pm    Post subject: Reply with quote
Well, this seems to be a trend these days. The revered Zeiss Otus 55/1.4 is a Distagon design if I'm not mistaken? They claim this leads to better sharpness distribution across the field and minimizes aberrations.


PostPosted: Tue Jan 07, 2014 7:37 pm    Post subject: Reply with quote
I have large "reproduction" lens, used in photocopy machines for large format documents. It is very huge, front element diameter is 105mm and fully symmetrical - image enters and outs of same size. I've opened it and it is 6 element double gauss design. Specs are 497mm/F6.5, photos can be found here (not my specimen): http://tumanyan.ucoz.ru/photo/mom/43-2-0-0-2

So I have an idea, if I remove the one gauss set, and attach smaller "rear" one from some 35mm lens, considering the large "entry" diameter of this lens, if I choose the attachment point properly, is there chances for me to get something like 300mm/F2.8 ?


PostPosted: Wed Jan 08, 2014 3:20 am    Post subject: Reply with quote
sichko wrote:
MacTak wrote:
Forgive me for dredging up an old thread, but I thought this would be the perfect thread to make some mention (and hopefully get a little discussion) of the new Sigma 50mm f1.4 Art lens announced today. For it features a very strange and (I think) novel design: the first quadruple-Gauss lens!?



Let's start with the rear half of the lens, which would itself be a relatively expected design for a fast 50--a Biotar/Xenon type, though one with a particularly enhanced rear group (two elements in the rear has been done on some fast 50mm, but here one of those elements is turned into a doublet and the other is aspherical). This rear half is much the same as the entire prior Sigma 50mm f1.4 DG EX lens (including that enhanced rear group with doublet + aspherical), except that lens split the front doublet (Ultron-type). But then there is also the whole front part of the lens design, and guess what, it clearly looks like another double-Gauss feature (stripped of any extra elements besides the opposed Gaussian elements themselves). In this case the front group remains split, and the rear group, instead of being the usual doublet (or split), has been made into a triplet! Like I said, pretty crazy.


There's a strong negative element in the front half, and the back half is positive and Double-Gauss-like. A retrofocus lens ?

It is a Double-Gauss (last five groups) based retrofocus . FYI, the much older Voigtländer Skopagon 2.0/40 has a 9e/6g Double-Gauss design too.


PostPosted: Wed Jan 08, 2014 10:46 am    Post subject: Reply with quote
danfromm wrote:

The Sigma thingy is strange. No way are the first three groups a double Gauss type, the third group is a more or less a single Dagor (or clone) cell. The rearmost group is a little hard to read, not clear whether it contains a cemented doublet, air space, and cemented triplet (Dagor-like, again) or is five elements cemented together. Either way, not half of a conventional fast double Gauss type. Calling the lens a quadruple Gauss type is a large stretch..


Based on Sigma's stated 13/8 design (sorry I had forgotten to put that in my prior post), it is clear that (from the rear) it is single aspherical element, air space, cemented doublet, air space, cemented doublet (which is the same as their prior 50mm f1.4 from the rear, by the way). Good point about the Dagor in the third group, but the first two groups still look Gaussian to me (given their shape and proximity), so I guess Sigma is replacing the rear of an opposed Gaussian with a Dagor to better correct some aberrations (and they can get away with this since they have more space afterwards to correct remaining abberations)? Hence the spacing to the third group.

By the way, I agree it is a double-Gauss based retrofocus (which has indeed been done, though less common than a retro-telephoto design), but my particular interest is that the front bit of it is more than just a few negative elements; the first two are indeed negative, but the third group (correct me if I am wrong) should be positive. The Zeiss Biogon spoilerhead has helpfully provided seems a little closer in thought to that than the Zeiss Otus. It still seems to me that Sigma is taking a rather novel approach to the design.

Thanks to everyone for the replies; I really know very little about optical design and am eager for a chance to expand my knowledge via a discussion like this!

Edit: It might be helpful to bring in another recent fast 50, the Pentax DA55/1.4.

Pentax has always liked the Ultron-type double-Gauss for their standard lenses, and three elements at the rear is not unheard of either. What is unique here is the element between the opposed Gaussian groups, which thus makes the two groups after the aperture into somthing resembling a Plasmat cell. So might this be another case, like in the forward portion of the Sigma, of upgrading the rear of an opposed Gaussian (Sigma to a Dagor cell, Pentax to a Plasmat cell)?

Last edited by MacTak on Wed Jan 08, 2014 3:04 pm; edited 1 time in total


PostPosted: Wed Jan 08, 2014 2:10 pm    Post subject: Reply with quote
MacTak wrote:
.... but the first two groups still look Gaussian to me (given their shape and proximity)....


What do you mean by Gaussian ? Wikipedia ( http://en.wikipedia.org/wiki/Gauss_lens ) tells us ...

The Gauss lens consists of two lenses; in its most basic form, a positive meniscus lens on the object side and a negative meniscus lens on the image side. The power of the positive element predominates...

Yes - I know this is Wikipedia, and there are no links to authorative sources. However it fits my own experience (ignorance?). In most of the pictures of Gauss lenses that I have come across, the front positive element seems to be stronger than the inner negative element. In the new Sigma lens it appears to be the other way around - with a weak positive element in front of a strong negative element. Can we still call this a "Gauss" ?

Quote:
... or we can simply think of the first two elements as negative elements....


The front element appears to be positive.


PostPosted: Wed Jan 08, 2014 3:02 pm    Post subject: Reply with quote
MacTak wrote:


Quote:
... or we can simply think of the first two elements as negative elements....


The front element appears to be positive.


You're right, the front element is positive, the second element is negative (now fixed). Indeed, that it's only that second element/group in that front section of the lens that is negative is why I do prefer not to think it is particuly helpful to think of this lens as a retrofocal design.

I think it's still fine to talk in terms of double-Gauss (the term is still used even when not a symmetric design, for instance), however it does become a bit of a question of semantics. To me, the question is what did they work from and modify.


PostPosted: Wed Jan 08, 2014 5:40 pm    Post subject: Reply with quote
sichko wrote:
MacTak wrote:
.... but the first two groups still look Gaussian to me (given their shape and proximity)....


What do you mean by Gaussian ? Wikipedia ( http://en.wikipedia.org/wiki/Gauss_lens ) tells us ...

The Gauss lens consists of two lenses; in its most basic form, a positive meniscus lens on the object side and a negative meniscus lens on the image side. The power of the positive element predominates...

Yes - I know this is Wikipedia, and there are no links to authorative sources. However it fits my own experience (ignorance?). In most of the pictures of Gauss lenses that I have come across, the front positive element seems to be stronger than the inner negative element. In the new Sigma lens it appears to be the other way around - with a weak positive element in front of a strong negative element. Can we still call this a "Gauss" ?

Quote:
... or we can simply think of the first two elements as negative elements....


The front element appears to be positive.


There are two possibilities mentioned in lens books, one is called Gauss type I and the other Gauss type II in which the sequence is reversed. Nevertheless both are Gauss type lenses.


PostPosted: Wed Jan 08, 2014 6:03 pm    Post subject: Reply with quote
MacTak wrote:
MacTak wrote:


Quote:
... or we can simply think of the first two elements as negative elements....


The front element appears to be positive.


You're right, the front element is positive, the second element is negative (now fixed). Indeed, that it's only that second element/group in that front section of the lens that is negative is why I do prefer not to think it is particuly helpful to think of this lens as a retrofocal design.


Have a look at this patent : http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=355452A&KC=A&FT=D&date=19310827&DB=EPODOC&locale=en_pl

It's from 1931 and it's by Horace Lee of TTH. If you look at the first picture on page 5 you can see that the lens design has a Back Focal Distance is longer than the focal length. Many people (see, for example, this Zeiss article : http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=355452A&KC=A&FT=D&date=19310827&DB=EPODOC&locale=en_pl ) would call this a Retrofocus lens. Of course Lee didn't call it that. The name was invented by Angénieux some 20 years later.

The arrangement of the first two elements of the new Sigma lens clearly resembles that of the first two elements in the 1931 Lee lens.

Quote:
To me, the question is what did they work from and modify.


Well, the 1931 Lee lens looks like a useful starting point. If you tweak the Double-Gauss relay at the back the essence of the design is unchanged. But what about the cemented triplet - the Dagor if you want to call it that - what does that do? Maybe it's just another "relay" with the introduction of three more elements adding another dozen or so degrees of freedom.


PostPosted: Wed Jan 08, 2014 11:05 pm    Post subject: Reply with quote
kds315* wrote:
sichko wrote:
MacTak wrote:
.... but the first two groups still look Gaussian to me (given their shape and proximity)....


What do you mean by Gaussian ? Wikipedia ( http://en.wikipedia.org/wiki/Gauss_lens ) tells us ...

The Gauss lens consists of two lenses; in its most basic form, a positive meniscus lens on the object side and a negative meniscus lens on the image side. The power of the positive element predominates...

Yes - I know this is Wikipedia, and there are no links to authorative sources. However it fits my own experience (ignorance?). In most of the pictures of Gauss lenses that I have come across, the front positive element seems to be stronger than the inner negative element. In the new Sigma lens it appears to be the other way around - with a weak positive element in front of a strong negative element. Can we still call this a "Gauss" ?

Quote:
... or we can simply think of the first two elements as negative elements....


The front element appears to be positive.


There are two possibilities mentioned in lens books, one is called Gauss type I and the other Gauss type II in which the sequence is reversed. Nevertheless both are Gauss type lenses.


Thanks Klaus. I didn't know. Is there on online source for this kind of information ?
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