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Double Gauss Designs
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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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