By Alexander Lee

Overview

This list is only for designs that the Carl Zeiss Foundation has some nominal claim on the origin of design.

I created this list for a few reasons:

• First because I was interested in the design criteria that Zeiss uses when building lens of different focal lengths, or same focal lengths but different speeds or physical size. As I felt it may have an impact on optical quality, I figured it would be good to know the family traits of lens designs for when I am shopping for new lenses.
• Second, there have been few groundbreaking lens designs since the Biogon was developed in the 1950's. Most modern lens are based on modified older designs, or older designs reworked with modern glass and multi-coating in mind. The most common lens design still is the almost 100 year old tessar type.
• Third, I thought it was a pretty interesting field that was not very well covered.

It was hard getting the data. Apparently there are some good books out, but my local library doesn't have them, and I'm too lazy to find it in other library systems have them request it via interlibrary loan. Most of my data came from "classic" large format lens FAQ's and various Contax/Hassleblad/Rollie lens propaganda, though the propaganda really only touted the praises in vague terms, and didn't mention any potential design limitations.

Recently, I picked up Rudolf Kingslake's excellent book "A History of the Photographic Lens", ISBN 0-12-408640-3 and am in the process of revising my data. Hopefully, I will also get Rudolf Kingslake's Modern Photographic Lens Design book as well, but I already have more material to work with than I have time to deal with it.


Misc. background on CZ lens

Bausch and Lomb was licensed to produce CZ lens in America for a while. They developed and acquired patents for several variants of CZ lens designs. Most patents back then were good for 17 years, though during the years when the USA was at war with Germany, the assets of German companies in America were put into a federal trust which managed the property, real and intellectual, until the end of the war. During that time, the federal agency could allow American companies to use the property for the war effort, or they could lease the property out. A notable example was the Leica New York repair facility. The government trust originally wanted Kodak to assume control of the repair facility to produce Leica copies using the tools and dies. Kodak wisely turned the offer down, avoiding legal problems after the war, as well as the fact that it was potentially (and actually turned out to be for the company that agreed to the offer) a money loosing proposition.

Ernst Abbe was hired by Carl Zeiss in 1866 to help Zeiss manufacture instruments in a proper scientific basis. in 1880, he hired Otto Schott to help develop new types of glass. They established Jena Glassworks in Jena Germany, and by 1886, they had a catalogue of 44 types of glass, many of which were new. They manufactured the first successful high-index crown glasses. These new glasses allowed lenses to be made that had a flat field free from astigmatism.

The Tessar design was produced in 1902, and awarded a US patent in 1903. When the patent expired in 1920 there were many variants on the Tessar design released other companies, such as some of Kodak's Ektars and Schneider's Xenars. The famous Leica 50mm f/3.5 Elmar lens released in 1920 was a Tessar type lens. After Leica introduced the first 35mm camera in 1925, Zeiss's camera division knew that they were behind in small format camera development responded a year later by acquiring four camera manufacturers a year later in 1926, Ica, Contessa-Nettel, Ernemann and Goertz and merging them into Zeiss Ikon AG.

After Zeiss absorbed Goertz in 1926, the American Goertz division became an independent company (Goertz American Optical Co.) and continued to make Goertz lens, as well as innovate new lenses. Goertz American Optical Co. was renamed to Goertz Optical Co. Inc. in 1964, purchased by Kollmorgen in 1971, then in turn, Kollmorgen was purchased by Schneider in 1972.

The first commercially viable lens coating process, but depositing a thin layer of low index material (originally calcium fluoride or magnesium fluoride) was developed by A. Smakula of Zeiss in 1936. Zeiss did not invent the concept and function of lens coating as the Contax website history wording would lead you believe. H. Dennis Taylor in 1896 observed that old lenses that had become tarnished by exposure (a natural bloom coating) transmitted more light than a newly polished lens. He postulated that the layer of tarnish had a lower refractive index than the glass, and therefore reflected less light and transmitted more if it. He was awarded the first patent for lens coating in 1903 for a chemical or acid fuming process that was highly unreliable, and the components very caustic. Viable lens coating allowed Zeiss's (and everyone else's) more complex lens designs to be implemented with much less flare and greater contrast. Previously those designs, while very much corrected for aberration and astigmatism suffered very poor contrast. The lens coating technology was considered a national security secret during W.W.II, and did not really become available to consumers until after the war was over. This coating though was a single layer that usually reduced the reflectivity of one wavelength of light. Light of other wavelengths were less affected the further up or down the spectrum from the prime wavelength. Later, plans to deposit multiple layers of coating allowed a more even reduction of reflectivity through the entire spectrum of photographical light. Leitz was the first company to sell a regular production multicoated lens to general consumers.

After W.W.II, the Allies brought the heads of the Carl Zeiss foundation, as well as key Zeiss foundation personnel from Dresden to West Germany, because they recognized the value of the optical knowledge and research that Carl Zeiss had, as well as the cold war desire for the technology not to fall into Communist hands. The Carl Zeiss foundation was moved to Western Germany (legal paper work here, not equipment) from Dresden as well. This was important because later, it allowed the Carl Zeiss Foundation to successfully press claim to various trademarked names, such as Zeiss, Carl Zeiss, and Contax. The Schott glassworks was slit into Jena Lenswork, in Russian hands, and Schott in Oberkochen.

The lensworks and camera factories in Dresden were destroyed in the Firebombing of Dresden February 14, 1945, but the Jena camera works continued to produce lens and cameras under the Carl Zeiss name until the trademark dispute between the East Germany company, and the West German was resolved. Some of the existing machinery was disassembled by Russian troops for war reparations, and were re-assembled in the newly forming USSR. The East German Carl Zeiss continued to produce cameras and lens from existing stocks of parts. Later they released a new line of SLR cameras labeled Carl Zeiss Dresden, the Contax S and Contax D. Eventually after the trademark dispute was settled, the eastern Contax SLR was renamed the Pentacon. Into the 70's, the East German Carl Zeiss had lens produced in Japan for several systems labeled Under License from Carl Zeiss Jena.

Since the re-unification of Germany, the Eastern and Western Zeiss companies have been re-united, and the headquarters is being moved back to Jena. The massive Jena optical factory which at it's height in Eastern Germany employed over 60,000 factory workers has been broken up and parts have been sold off. The binocular section has been sold to Dr. Optik, and Schneider has purchased part of the Glassworks.

Symmetrical lenses

Meniscus lens with a stop in front exhibits barrel distortion, if turned around with the stop behind the lens, it reversed into a pincushion. If two were mirrored around the stop, the distortions cancel each other and leave a distortion-less system. It also corrects for lateral color (chromatic difference of magnification) and coma. Truly symmetrical lenses must be used at 1 to 1 magnification to totally cancel the three aberrations, and exhibit some very small amounts of transverse aberration at infinity.

Asymmetrical lenses

Asymmetrical lenses, while not totally corrected for those three aberrations, can allow larger apertures and cover wider fields than symmetrical designs.

Dialyte - Airspaced Achromatic doublet.


Lens Designs

Some definitions of terms I use in the list

• Aberrations - Aberrations are image defects that result from limitations in the way lenses can be designed. Better lenses have smaller aberrations,but aberrations can never be completely eliminated, just reduced. The classic aberrations are:
  
  • Spherical aberration. Light passing through the edge of the lens is focused at a different distance (closer in simple lenses) than light striking the lens near the center.
    
  • Coma. Off axis points are rendered with tails, reminiscent of comets, hence the name. It can be shown that coma must occur if the image formed by rays passing near the edge of the lens has a different magnification than the image formed by rays passing near the center of the lens.
    
  • Astigmatism. Off-axis points are blurred in their the radial or tangential direction, and focusing can reduce one at the expense of the other, but cannot bring both into focus at the same time. Think of it as the focal length as varying around the circumference of the lens. (Optometrists apply the word "astigmatism" to a defect in the human eye that causes *on-axis* points to be similarly blurred. That astigmatism is not quite the same as astigmatism in photographic lenses.)
    
  • Curvature of field. Points in a plane get focused sharply on a curved surface, rather than a plane (the film). Or equivalently, the set of points in the object space that are brought to sharp focus on the film plane form a curved surface rather than a plane. With a plane subject or a subject at infinite distance the net effect is that when the center is in focus the edges are out of focus, and if the edges are in focus the center is out of focus.* Distortion (pincushion and barrel). The image of a square object has sides that curve in or out. (This should not be confused with the natural perspective effects that become particularly noticeable with wide angle lenses.) This happens because the magnification is not a constant, but rather varies with the angle from the axis.
    
  • Chromatic aberration. The position (forward and back) of sharp focus varies with the wavelength.
    
  • Lateral color. The magnification varies with wavelength.
    
    
• APO or Apochromatic - The distance behind the lens at which monochromatic light (light of a single wavelength) comes to focus varies as a smooth function of the wavelength. If this function has a zero derivative in the visible range, and hence if there are two wavelengths at which the light comes to focus in the same plane, the lens is called achromatic. If there is a higher order correction, usually with the result that 3 or more visible wavelengths come to focus at the same distance, the lens is called apochromatic. Some authorities add more conditions. Apochromatic lenses often contain special low-dispersion glasses. APO is an abbreviation for apochromatic.
  
• Asymmetrical - Front and rear lens groups of lens are not the same.
  
• Cells - Sets of lens groups designed to function as a unit.
  
• Circle of good definition - Effective circumference of coverage that is usably sharp.
  
• Circle of illumination - Effective circumference of coverage that has useable/recordable light.
  
• Coating - Before coating, each transmission surface resulted in about a 4% to 8% loss of light to reflection depending on the refractive index of the glass. So an uncoated Dagor or Protar with four transmission surfaces looses 15% to 29% of the light to flare. An uncoated Tessar looses 22% to 40% of the light to flare. An uncoated Planar with eight surfaces looses 28% to 49% of light to flare. The flare would exhibit itself on the film as unfocused non-image forming light which reduced the contrast of the picture.
  
  • Single Coating - After single coating, this dropped to about 2% to 4% loss of light per transmission surface. Applying the coating at quarter wavelength thickness could greatly increase the effectiveness of the coat, but it could completely block some wavelengths of light and partially block others. Typically blue-green wavelengths were suppressed with an amber coat, or green wavelengths with a purple coat.
  • Multi-coating - Multicoating was first done as two separate coats at different wavelength thickness on different transmission surfaces to balance the color of the light transmitted to the film. Later, multi-coating as we know it, one coat stacked on another (first used on a production lens by Leitz) reduced the light lost to diffraction further to about 1/2% to 1% per transmission surface. The classic second coat was bismuth oxide again applied at quarter wavelength thickness for a different wavelength, typically orange-yellow for the second coat and green-blue for the first coat giving a faint green reflection. A multi-coated Planar could now only loose about 4% to 8% of the light to flare, quite a difference.

Coating and multicoating allowed designers to use more complex designs with more air spaces which allowed easier design for correction of spherical aberrations. The difference between uncoated lenses and coated lenses are great, the difference between single coating and multi-coating is visible, but not nearly as great as the first leap from uncoated to coated. Coating and multicoating opened the way for many otherwise unfeasible modern lens designs, such as complex wide-angle lenses, big multi-element zooms, and lots of marketing hype. Coating still won't save you from nasty flare in certain lighting conditions, such as shooting into the sun, so make sure to use those lens shades!

• Convertible Lens - A set cells that can be combined together in different pairs or singly to produce different effective focal lengths. Each cell must be able to correct and focus the image properly on the film alone. If a cell is used singly, it is mounted behind the shutter.
  
• Elements - individual lens
• Groups - single lenses or groups of lenses cemented together
• Symmetrical - Front section of lens is identical mirror image of rear section. Because of this, aberrations and astigmatisms are minimized. Inherently optimized for 1:1.
• Transmission surfaces - Lens element to air surface.