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Daniel Otranto
26-Apr-2007, 21:08
i have an enlarging lens of 160mm on my 4x5 but in order to put it on the lensboard, the distance between the front glass and the back glass on the lens is made slightly longer, how will this effect the image or focus?

JW Dewdney
26-Apr-2007, 23:17
It seems you would be in a far better position to tell us what the effect is...(?)

Uli Mayer
27-Apr-2007, 00:38
By altering air-spaces you are tinkering with lens design. Just think of air-spaces as lenses made of special glass with refract.index 1 (=air).

Bill_1856
27-Apr-2007, 01:00
With Tessar design lenses, increasing the space by screwing out the front element decreases the focal length for closer focus. (That's how many cameras focus, such as all the Zeiss Ikontas, Kodak Tourists, etc).

Daniel Otranto
27-Apr-2007, 06:44
It seems you would be in a far better position to tell us what the effect is...(?)



i have not shot with this set up, i was wondering if anyone had a quick response to see if it was worth shooting with it or not

thanks

dan

Ole Tjugen
27-Apr-2007, 08:05
With Tessar design lenses, increasing the space by screwing out the front element decreases the focal length for closer focus. (That's how many cameras focus, such as all the Zeiss Ikontas, Kodak Tourists, etc).

That's because nearly all the "focal power" of a Tessar is in the front cell. The rear cell is mostly correction, and has very little "power". So by unscrewing the front cell you increase the extension. The focal length changes very, very little if at all.

You will also get less quality, since the corrections are no longer optimal.

Ernest Purdum
27-Apr-2007, 09:14
I havew personal experience with this, caused by a lensboard a little too thick to allow the rear cell to fully screw in. The result was soft focus.

John Bartley
27-Apr-2007, 10:19
No answer here, but I am curious ... if the enlarging lens came off a lens board, why would it not fit properly on another lens board? Maybe the answer is not to shoot with it improperly mounted but rather to redo the mount?

cheers

Uli Mayer
27-Apr-2007, 10:51
Ole,
for a better understanding on how Tessars (and other triplet modifications) actually work, you may read this:http://www.willbell.com/tm/The%20Cooke%20Triplet%20and%20Tessar%20Lenses.pdf

From the viewpoint of aberrations and corrections, it's completely irrelevant how a lens is split up into cells; cells are nothing but mechanical means for arranging the lens on both sides of the diaphragm.

Whatever you mean by "focal power" ( a strong one in front, a weak one at the rear) please do not forget that - though seemingly otherwise - the Tessar still contains much symmetry. If FLUEGGE 1) is right, it is the double-concave lens in the center that has the largest share in correcting aberrations. And, as this thread is about air-spaces, let me just quote Willy Merté:

"Der Tessartyp enthält vier Linsen, zwei äußere sammelnde und zwei innere zerstreuende; eine von diesen ist von der benachbarten Sammellinse durch eine zerstreuende Luftlinse getrennt, die vor allem für die sphärische Korrektion wichtig ist; die andere Zerstreuungslinse ist von der benachbarten Sammellinse durch eine sammelnde Kittfläche getrennt, die vor allem für die Beseitigung schiefer Bündel wirksam ist." 2)

No way to talk about Tessar design along th superficial category of lens cells.

1) Johannes Flügge "Das Photographische Objektiv" (Die wissenschaftliche und Angewandte Photographie? Springer-Verlag, Wien, 1955 p. 301

2) W. Merté, R. Richter, M. v. Rohr: same title and publisher, Wien 1932 p. 283

JW Dewdney
27-Apr-2007, 12:33
i have not shot with this set up, i was wondering if anyone had a quick response to see if it was worth shooting with it or not

thanks

dan


Daniel - the point I was (subtly) trying to make is that maybe you shouldn't rely on the subjective views of others! You may find a technique or a photograph by simply trying it that you find life-changing! You never know. Perhaps it's a soft-focus effect - or perhaps that configuration is absolutely incredible at 1:1 magnifications. There are subtelties and surprises in every lens design, and in every eye...!

Ole Tjugen
27-Apr-2007, 22:29
No way to talk about Tessar design along th superficial category of lens cells.

Sometimes a simplification makes the answer understandable to the person who asked. It may not be correct, but sometimes a gross simplification can be better than a completely correct answer.

One could of course plug all the data into OSLO, change the spacing, and see what happens (that's what I would do if I really wanted a "correct" answer). :)

Doug Dolde
27-Apr-2007, 22:32
Same way the space between your ears affects your thinking.

Uli Mayer
28-Apr-2007, 00:38
Ole,
this is not a question about "clarity" ( broad brush strokes, simplication ..) versus "precision" (number of digits after the decimal point ...).
What I am saying, is, your Tessar explanation is neither clear nor precise: It is simply wrong and not to the point.

Ole Tjugen
28-Apr-2007, 01:11
The design and performance of a Cooke Triplet is the main subject of this
chapter. The construction of a Cooke Triplet is illustrated in Figure B.3.1.1. Also
included here for comparison is a Tessar lens, which can be considered a derivative
and extension of the Cooke Triplet (although Paul Rudolph designed the first Tessar
in 1902 as a modification of an earlier anastigmat, the Protar).

If the Tessar is considered as a modified Protar, what I wrote is correct enough. Consider it as a modified Triplet, and it's not correct.

Yet both viewpoints are compatible with the observation that many front-focusing Tessar-type lenses focus by moving the front (air-spaced) pair; while others move the front element only.

Uli Mayer
28-Apr-2007, 03:41
Whatever led to the Tessar's development (its origin can as well be traced back to the "Portrait-Antiplanet" (Steinheil), or to Taylor's triplet, or to the "Protar-Unar" line,as preferred by Zeiss historians) - it's irrelevant here.

It's about getting it right how a Tessar actually works. By separating "focal power" and correction of aberrations in the way, Ole did , the final interplay of all lens parameters is grossly misrepresented.

But let's stop talking about lens designers' tool-boxes I am only pleading for paying more respect for those designers' work. It involved much more "hair-splitting" and tearing one's hair than probably anyone here would like to do. I can't handle those tools myself. And with regard to splitting and tearing hairs, I have to admit I am bald.