Symmetrical Or Not?

[JOSEPH ANDERSON]

Hi all, this is for you glass gurus. I normally use a componon-s for flat art copy. But, last week I only had one piece to copy. Since I had a 203 ektar on a 4x5 crown. I though I would try it and see what I got. The original is a black & white stipple 22 by 22 in. I made it about 3.5 in square on the negative. Tmax 100 pushed 1 stop so it can stay in developer a little longer. I don't know if this is the correct thing to do or not but,it seems to work. my friend asked why I didn't reverse the lens. I thought the 203 ektar is symmetrical and it would not matter. he said they are almost symmetrical and it would improve the image. I am not a glass guy so I couldn't argue the point. I'm sure many of
you know if the 203 is symmetrical or not. Am I wrong as usual, or do I win one for a change. BTW the negative and print came out great. although the prints were only 10 in square. so no real test there.
Thanks all, Joe A

[Paul Fitzgerald]

Joe A,

you won, according to the Kodak specs it is symmetrical.

[JOSEPH ANDERSON]

Thanks Paul,

that's one for me so far.

[aduncanson]

Is the 203 Ektar exactly symmetrical? I don't know. In the case of many lens designs the word "symmetrical" really only describes the intellectual basis or point of departure for the lens design and those who interpret the word literally are ill served.

In any case, it only makes sense to reverse an unsymmetrical lens when a lens (like a typical taking lens) is optimized for reproductions smaller than 1:1, and is used to make reproductions greater than 1:1 - or vice versa. You used a taking lens to make an image about 1/7th the size of the original. You were a long way from benefiting from reversing the lens.

[FilmIsNotDead]

Symmetrical lens design only means the same number of elements in the same number of groups either side of the aperture. Shapes and spacing can be tweaked to better perform particular tasks. A Tessar-type lens optimized for near infinity will perform better reversed when used for close-field work. Whether or not the difference is worth the effort is lens/subject dependant and a matter of personal taste/opinion.

[Dan Fromm]

"Symmetrical lens design only means the same number of elements in the same number of groups either side of the aperture."

What is your native language?

[JOSEPH ANDERSON]

Thanks all, guess I'm only half right. I thought symmetrical were the same front and back. Dan it's english. is my grammar that bad? I know my typing and spelling is.

Thanks again for the info

[Dan Fromm]

Joe, I didn't ask you your native language, I asked the person hiding behind FilmIsNotDead.

You asked a sensible question whose answer has practical implications, namely, whether to reverse a 203/7.7 Ektar when shooting at magnifications below 1:1. Short answer, true for asymmetrical lenses too, is "no." Above 1:1, "yes." For symmetrical lenses, which end of the lens faces the subject doesn't matter regardless of magnification.

I'm at work. After I get home I'll ask my 203/7.7 what it thinks. If it is symmetrical, front and rear cells will have the same focal length. If not, not.

FWIW, the cross section that EKCo published in its propaganda shows a lens that looks symmetrical, i.e., with (in this case) the two outer elements having the same radii and thickness and the two inner elements having the same radii and thickness. This isn't conclusive because published cross sections don't always match prescriptions. Eric Beltrando (see http://www.dioptrique.info/base/m/m_kodak.HTM), who worked from US Patents, shows two slow Kodak dialytes that are absolutely symmetrical. So the 203/7.7 certainly could be symmetrical.

With respect to the definition of symmetrical provided by the person hiding behind FilmIsNotDead, I have a small heap of 6/4 plasmats whose cells have quite different focal lengths. Calling them symmetrical is silly.

Cheers,

Dan

[FilmIsNotDead]

Dan,

My native language is English. Did I say something offensive?

[Dan Fromm]

F.I.N.D., not to discourage you from posting, but y'r answer to the OP's question struck me as silly and perhaps a little wrong. Not offensive.

The OP's question was about proper orientation of a lens when shooting below 1:1. Your comment about symmetry doesn't address it. It also seems silly. One can find, in fact I have, lenses that are, by your definition, symmetrical but whose orientation matters.

If you mean "at magnifications > 1:1" when you say "close-field work" then yes, you're right, asymmetrical lenses optimised for distant subjects should be reversed. If your "close-field work" means "at magnifications <= 1:1" then your advice is wrong. Common error, if you made it.

People whose native language isn't english often produce, with the best of intentions, utterances in english that seem silly to native speakers. It doesn't do to chide them for making an effort to communicate with us. But native speakers themselves are fairer game. That's why I asked.

Cheers,

Dan

[aduncanson]

I am thankful that Dan did not question my parentage or my native language since my definition also would class his plasmats as symmetrical. I think that classification is not silly, but helpful as long as "symmetrical" is not understood literally.

While not everybody's favorite lens guru, Arthur Cox was an accomplished lens designer; head of optics at Bell & Howell and an author of books on optics. In Photographic Optics He classified lenses according to the designer's thought process. If the designer started from the knowledge that a symmetrical arrangement about the aperture eliminated many aberrations at 1:1 and controlled them well over a broad range of reproduction ratios, then Cox would class that design as symmetrical, no matter how unsymmetrical the eventual configuration of elements turned out to be. In this category Cox places Dagors, Planars, and plasmats but not dialytes like the 203mm Ektar. In fact he consistently classes the dialytes as being derived from the triplet design, possibly derived by splitting the triplet's middle, negative element with the aperture.

There are certainly instances where the designer's thought process was not well documented and Cox made inferences that others may dispute. However, I have always found Cox's approach to be useful, and (silly me) I thought it was conventional.

None of this addresses your original question, except to warn you that the word "symmetrical" as used in a lens brochure or an internet discussion group may not literally mean symmetrical as you learned the word in a math class.

If Dan insists that "symmetrical", in the context of lens design, must mean literally symmetrical then I will allow him his usage and hopefully avoid conflict by making clear that my usage is more metaphorical.

[Dan Fromm]

A, I know that some people think a poster's parentage is relevant to discussions here. I've never understood that position. Unwed parents, mother of the canine persuasion, child or practitioner of incest, who cares?

In the context of photography at magnifications above 1:1, whether a lens is truly symmetrical makes a difference. This is why I don't go along with loose use of the term even though I have a well-worn copy of Cox' Photographic Optics.

Incidentally, even though I'm an, um, strict constructionist, I have a little 6/4 plasmat type that I shoot facing normally at magnifications from 1:8 to 2:1. I've tested it from 1:8 to 4:1, it is superb over that range. And I've asked it whether it is truly symmetrical. It isn't. My only better lens over that range is a reversed tessar (reversed in the ordinary, not Kingslake, sense, i.e., with the cemented doublet facing the subject). So much for theory.

Oh. What lenses? 4"/5.6 Enlarging Pro Raptar, 100/6.3 Neupolar.

Cheers,

Dan

[Diane Maher]

Joe A,

you won, according to the Kodak specs it is symmetrical.

Paul,
Where can the Kodak specks on this lens be found?

Diane

[GPS]

Lenses are sometimes also called symmetrical when their exit pupil diameter is equal to their entrance pupil diameter. This can be a case even for lenses that are otherwise not symmetrical in their optical design.

[Ernest Purdum]

With all appreciation of Arthur Cox and other learned individuals who find it convenient to construct special definitions of the word "symmetrical", I think there is less apt to be confusion if the common definition is used.

[GPS]

With all appreciation of Arthur Cox and other learned individuals who find it convenient to construct special definitions of the word "symmetrical", I think there is less apt to be confusion if the common definition is used.

I agree with you. The science of optics is not limited along native language lines, otherwise it would not be a science. Symmetrical is symmetrical regardless of your native language. Not regardless of your technical education though. :)

[Paul Fitzgerald]

Diane,

"Paul,
Where can the Kodak specks on this lens be found?"

in the 'Kodak Reference Handbook', I'll scan it later and post it

[Dan Fromm]

Paul, I'm sorry but I can't get mine apart so couldn't check the two cells' focal lengths.

Cheers,

Dan

I think the scans you're making are posted on Brian Wallen's site, see http://www.bnphoto.org/bnphoto/KA_KASDocPro.htm and http://www.bnphoto.org/bnphoto/LD48_anas8.htm . IMO they don't prove much.

[JOSEPH ANDERSON]

thanks all for shearing your wisdom and expertise,regarding my question. Up until just over two years ago I didn't care how or why a lens did it's job. Then I stumbled, by chance on this forum. I rapidly developed a inferiority complex. Who are these people, optical engineers, photographers, both it seems.
I understand must of this info. Some of it though is over my head.

Joe A

[Dan Fromm]

Joe, your mantra for the day is "The image matters, not what people say about lenses."