----- Original Message ----- From: "Ardeshir Mehta" <ardeshir@xxxxxxx> To: <rollei_list@xxxxxxxxxxxxx> Sent: Wednesday, May 25, 2005 8:09 PM Subject: [rollei_list] Re: Planar, Xenotar, Summicron > > On 25-May-05, at 7:24 PM, Richard Knoppow wrote: > >> [...] >> >> In any case, all of these lenses, five element and six >> element >> Planar/Xenotar, and the Leitz Summicron, are all members >> of the >> same family of lenses. > > MOST instructive. Thanks. > > BTW: does anyone (Richard?) know to which family the > (modern) Canon > EF 50 mm f/1.8 lens belongs? (I'm NOT speaking of the 50 > mm f/1.4 here.) > > (I'd like to know since I have one of those for my EOS > Elan 7, and it > comes highly recommended, optically speaking, even though > it's quite > inexpensive). > > Cheers. I don't know. If its on a web site somewhere I will look at it. However (a big one) Kingslake points out that in these days of computer design some modern lenses are difficult or impossible to classify as being derived from one of the classics. Even some old lenses can be thought of as either of two designs or maybe more. For instance, the classic aerial survey lens the Zeiss Topogon, designed by Robert Richter, four elements all deeply curved meniscus. Is it a double Gauss lens or is it a compounded Goerz Hypergon? Well, its really both. The Hypergon, for those not familiar with it, is an extremely wide angle lens with coverage of over 130 degrees, designed in 1900 by Emil von Hough, the designer of the Dagor. This lens consists of two very steeply curved meniscus elements on either side of a stop. The elements are very thin and the outer surfaces nearly form a sphere. The lens has a very flat field and very little astigmatism, and low coma and distortion due to its symmetry. However it is not corrected for spherical or chromatic aberration so can be operated only at very small stops, less than f/20. The fall of of illumination is even more than the rule of thumb cos^4 theta so the lens was equipped with a spinning obstructive stop to even it out. The Topogon has four elements, the outer ones positive thin meniscus as in the Hypergon but the inner ones are negative meniscus lenses as in a double Gauss type. The additional elements allow it to be corrected for spherical and chromatic aberration. The Bausch & Lomb Metrogon has an additional element which further corrects the spherical. What kind of lenses are the Topogon and Metrogon? As above they can be thought of as either double meniscus or as double Gauss lenses. There are more difficult cases in some modern lenses, for instance, few zoom lenses can not be classified as being derived from any of the older types, they are just their own thing. What is interesting is to learn how the various aberrations are corrected in the different types and what tricks the designers found to correct them. For instance, one trick used by Bertele in the Ernostar and Sonnar was to use thick, low index, sections instead of air spaces. The advantage of this was the elimination of flare while retaining some of the benifits of the air space. Paul Rudolf found a way of using a cemented interface to vary the dispersion of the cemented pair virtually at will without having any effect on other optical characteristics. He used this trick, called a "buried surface" in the original Planar to get the effect of a glass type which was not obtainable. Bertele uses the same trick in the f/1.5 Sonnar. Another trick, already mentioned, is the splitting of a strong element up into two or more weaker elements. Simply splitting them reduces some aberrations which is helpful when the angles of incidence in the lens become large as in very fast lenses or wide angle lenses. Because most of these tricks can be adapted to any design they are not really a basis for classifying a lens even though the trick may have originated with a particular type or be characteristic of it. Computer analysis of designs has made a huge difference in design technique. The method of evaluating a design is the trace rays of light through it. About three rays are necessary to get any idea of what its doing. By hand methods a single ray trace will take perhaps half an hour. If a hand calculator is used this can be reduced to perhaps five minutes. Any of the common computer optical design programs (OSLO, Zemax, etc.,) operating on a fast PC, can make millions of tray traces in a fraction of a second. Its possible to get a very complete analysis of a prospective design very quickly and to derive presentations of the information which were not practically possible before computers. The ray tracing is so fast that the computer program can be set to vary certain parameters to optimise the design, but as Kingslake and Warren Smith point out the program can't always tell when it is getting into impractical areas so it needs human guidance. This is not to say that all old designs were less than optimum. Brian Caldwell, a well known lens designer and the author of the program LensVIEW, says than many of the old Zeiss designs are so close to optimum that computer optimisation, even with changes in glass to modern glass, does not improve them significantly. This is partly due to very careful calculation but also because the old method of design was to evaluate the presciption mathematically until it looked pretty close and then build a model of it. Optimisation was then done by poking at the actual lens until it performed as well as could be gotten. Some advantages of modern design are less significant than might be thought. For instance aspherical surfaces have been around for a long time. Zeiss used them in some experimental lenses included in the survey of lenses called the Zeiss Index. An asphere can be duplicated by several spherical surfaces. The advantage of the asphere is simplification of the lens. Modern manufacturing processes allow economical production of aspheres. In the past each one had to be hand figured. Another advantage of the last sixty years has been the avialability of glass with very high indices of refraction and relativly low dispersion, or low index-high dispersion glass. All glass bends light. The amount it bends is related to the Index of Refraction. The idex is the bending of light compared to a vacuum, which has an idex of 1. Actually air is so close to 1 than it is usually considered to have an index of 1 except for the most precise work. The index of refraction is also the ratio of the speed of light in the medium to the speed in a vacuum. Now, things would be fine if the value of the index of refraction a constant. It isn't: it varies with wavelength, generaly going up as the wavelength decreases. The effect is known as dispersion. This is why a prism splits white light into a spectrum. The same effect is produced by simple lenses. In fact, two prisms, base to base, are an elementary positive lens. This effect is known as chromatic aberration. It is corrected by combining a positive element with a given amount of dispersion with a weaker negative element with abuot the same amount of dispersion. If the dispersions are nearly the same they will cancel. However, in order for this conbination to have any power the positive element (assuming we want a achromatic positive lens) must have more power than the negative lens. Practically, this means combining a positive lens with a high index but relatively low dispersion with negative lens with lower index but the same dispersion. Before Abbey and Schott came up with the Barium glasses known as Jena glass, in the late nineteenth century, all glasses followed a line where increasing index was accompanied by increasing dispersion. This meant that positive and negative lenses had to be assembled in a certain way to cancel the chromatic aberration. The disadvantage of this is that the reverse combination of positive and negative was needed to correct astigmatism. So that it was impossible (they thought) to make a lens of old glass that was both chromatically correct and free of astimatism. As it turned out, this was not true but a chromatically correct anastigmat of old glass was not produced until the 1920's by K. Martin of Busch (the Omnar). Even so the invention of the new type glasses furthered optical design enormously. The range of glasses was increased even more by the development of rare-earth glasses at the United States National Burea of Standards beginning in the early 1930's and developed by Eastman Kodak in the late 1930's. By increasing the index of a glass, especially if the dispersion can be kept low, the curvature of the surfaces for a given amount of power can be reduced. Since several aberrations are proportional in some way to the angle of incidence of light at the lens surfaces the glass automatically reduces the aberrations. This allow either an improvement in performance with a given amount of complexity or a duplication of performance with a simpler system. I've written too much. --- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx - Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org - Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field OR by logging into www.freelists.org - Online, searchable archives are available at //www.freelists.org/archives/rollei_list