View Full Version : Kodak 14" Commercial Ektar

11-Apr-2004, 17:41

I have a question about the 14" f/6.3 Kodak Commercial Ektar.

I am under the impression that it is a tessar-type lens.

Does anyone know the exact number of elements, and any other details, most preferrably a diagram of the lens?

Thank You Very Much, Jason Antman

Robert A. Zeichner
11-Apr-2004, 18:10
The 14" Commercial Ekatar is a 4 element, air spaced, symetrical, dialyt type. The outer elements are bi-convex, the inner elements, bi-concave. I hope that helps.

Michael S. Briggs
11-Apr-2004, 18:26
I have an original Kodak brochure that includes the specs of the Kodak Commercial Ektars. A cross-section diagram clearly shows a tessar-type design of 4 elements in 3 groups. Kodak states the coverage as 53 degrees at full aperture and 64 degrees at "small stops".

For the 14 inch focal length focused on infinity, these angles imply diameters of coverage of 14 and 17.5 inches.

Robert seems to be thinking of the some other Ektar, such as the 203 mm f7.7, which is a 4 element air-spaced dialyte.

If you have one of the lenses, an easy check can be made using a characteristic of the tessar design described by Kingslake on page 87 of his book "A History of the Photographic Lens" -- the front component of a tessar has very little power. So if you unscrew the cells, the rear one should function as a magnifier, while the front one will at most be a very weak magnifier. If a lens doesn't have these characteristics, it can't be a tessar. Having these characteristics doesn't prove that it is a tessar type. If the two cells are different, it can't be a symmetrical dialyte.

John Kasaian
11-Apr-2004, 18:51

The f/6.3 Commercial Ektar is a Tessar design and IMHO the pick of the (tessar) litter.

From Rudolf Kingslake's 'A History of the Photographic Lens'---

"Rudolph's patent was extremely general. His single claim reads:

"A spherically, chromatically, and astigmatically corrected objective consisting of four lenses separated by a diaphram into two groups each of two lenses, of which one group includes a pair of facing surfaces and the other a cemented surface, the power of the pair of facing surfaces being negative and that of the cemented surface positive."

FWIW, I recall reading somewhere that the 14" Commercial Ektar was a favorite of Karsh.

I sure like mine!

John Kasaian
11-Apr-2004, 19:06
The excerpt I quoted above regards the patent for the Tessar design. Michael S. Briggs statement that the Commercial Ektar is four elements in three groups suggests that it was an improvement over the original tessar, which might also explain why they are still highly valued by photographers.

Dan Fromm
11-Apr-2004, 19:28
John, I think you misread the text, which describes four elements in three groups. Cox gives the same diagram for Zeiss Tessars and tessar-type Ektars. The Commercial Ektars' big advantage over most other tessar-type lenses is that they are have larger coverage. Interestingly, the Vade Mecum rates early f/6.3 Zeiss Tessars above later faster ones.



John Kasaian
11-Apr-2004, 23:00

I got out my copy of Kingslake again. Four lenses in two groups according to designer Paul Rudolph. This is on page 87 of The History of the Photographic Lens by Kingslake, published by Academic Press, Inc. San Diego 1989(ex Cuyahoga County Public Library, BTW)

I admit that I'm operating under distress caused by my wife's black bean and garabanzo salad I had for supper, but I can't attribute that malady to Kingslake(the Dean of Kodak lens design) or Rudolph(der Zeiss-meister) being in error regarding the design.

Still, I'm certain that you and Michael S. Briggs must have a good idea about what makes up a tessar, but looking at the diagram in Kingslake's book I have to ask if you and Michael consider the faced(uncemented, air spaced group) to be not one, but two groups of lenses, each with one lens? That would make the lens having four elements in three groups, right? But Rudolph patent claim says the two elements make up one group, right? Am I missing something here? I'm confused and that bean salad ain't helping matters!;-) ---Cheers!

Michael S. Briggs
12-Apr-2004, 02:38
Yes, John, in the standard terminology, the tessar has four (optical) elements in three groups. The figure on page 87 of Kingslake's book shows two elements in two groups in front of the aperture, which is to the left of the aperture in the figure, and the right of the figure shows the rear group, consisting of two elements cemented together.

Rudolph's patent has fancy language, and for today, less standard terminology. He is using the word "group" in the normal way and not in the lens-construction-terminology way. The "four lenses" are what we are calling four elements. The phrase "two groups each of two lenses" isn't using the word group the modern way -- we might say a front portion of two elements in two groups, and a rear portion of two elements in one group. (In almost all real designs, the front portion would become a mechanical assembly or cell, and so would the rear portion.) His first "group" with "a pair of facing surfaces" is really a portion with two separated lenses/elements, and thus two groups in the modern terminology -- the "facing surfaces" implies that the surfaces aren't touching and thus we have two separated elements = two groups. The other group has a "cemented surface", showing that this is a group by the modern definition -- the lenses/elements are cemented into a single assembly.

John Kasaian
12-Apr-2004, 08:04

Thank you for the explanation. I see now. ----Cheers!

David Vickery
12-Apr-2004, 11:17
Hello All, Michael, I would like to "argue" with you about your last posting and the idea that a Tessar is correctly called a design of four elements in three groups. I believe that the original designers are more correct in their terminology than we are today because the front air-spaced group of the Tessar lens was made up of "a pair of facing surfaces..." with the "narrow airgap in the shape of a positive lens to correct the spherical aberration..."(p.86--Kingslake). I believe that they described the front group as one group of two elements in one group because it contained two lenses and an airgap of a specific size and shape that replaced the "cemented interface". It wasn't just an airspace like in a telephoto lens where the airgap is largely what determines the focal length but provides no corrective function for aberrations. Whereas the airgap in the Tessar was thought of as a corrective "lens" for better handling of the spherical aberration. So, even though Kingslake makes the statement "Rudolph's patent was extremely general" I believe that it is us that is using incorrect lens terminology for the description of the Tessar. Have I stated my argument well enough to convince you of this last assertion or do you think I am full of ____??? Of course if all you care about is making images with one of the best lenses ever manufactured then this argument of mine is kind of silly. The f/6.3 Tessars from Kodak are fantastic lenses. My version is an older 14" "Eastman Ektar" which I guess is the forerunner of the "Commercial Ektar". This is my favorite lens for 11x14. Jason, if noone has emailed you a scan of the diagram of the Tessar let me know and I will see if I can do that.

Ernest Purdum
12-Apr-2004, 15:37
Describing one piece of glass as a "group" is kind of silly, but that's lens-speak.

It should be noted that "Tessar type" lenses are by no means identical to the 1902 patent. They have been greatly improved and have come to have individual characteristics such as differing coverage, apochromatic correction, etc. so one more recent Tessar type isn't necessarily equivalent to another.

Robert A. Zeichner
12-Apr-2004, 17:26
My apologies for the misinformation in my previous post. A handbook in which I apparently placed too much trust has turned out to be wrong. I did look through some of my old Kodak literature and found a lens data book that diagrams the Commercial Ektar 14" as what appears to be a Tessar type formula. Four elements, the front two air spaced and the rear two cemented. Not symetrical either.