Bob Shell: Letters From Prison #28
Letters by Bob Shell, Copyright 2018
PHOTOGRAPHY & OPTICS
Most people know that light moves really fast. In ancient times it was believed that light was instantaneous, but as the science of physics developed it was realized that light does move at a measurable speed. That speed is about 186,284 miles per second in a vacuum. Light’s speed through transparent media is a bit slower, although I’ve never bothered to memorize what the speed is in various media. What’s important to know is that as light moves from one medium to another, say from air into optical glass its speed changes slightly. This phenomenon is what allows a lens to bend light to converge or diverge it. A lens that’s thicker in the middle and thinner toward the edges will converge light and is capable of forming a projected image. An ordinary magnifying glass is an example, and you can use it to project an image onto a surface. Conversely, a lens that’s thin in the middle and thick at the edges will diverge light and cannot form a projected image by itself. How much a piece of optical glass bends light is referred to as its refractive index, the higher the refractive index the more a ray of light is bent.
But that’s not the whole story. Everyone has seen how a prism breaks “white” light into its components. That’s where Mr. Roy G. Biv makes his appearance as an easily remembered mnemonic for the colors, called the spectrum. Red, Orange, Yellow, Green, Blue, Indigo, Violet. These colors we see are only part of the spectrum, which extends beyond red into infrared, and on to more energetic waves like X-rays. It also extends below violet into the ultraviolet. Insects and some birds (raptors in particular) can see ultraviolet, while most mammals see a narrower range than we do, being red-green color blind or monochromatic. It’s been speculated that primate color vision evolved to distinguish ripe fruit from unripe, but I’m not completely convinced by this proposition, partly because in some species of New World monkeys only the females have color vision. (Most of us have tricolor vision, with cells in our retinas sensitive to red, green, and blue, but a small percentage of us have four, although I’m not exactly sure what they can see that the rest of us can’t.)
Anyway, prisms made of different types of glass will spread the spectrum into wider or narrower bands. This differential spreading of colors is referred to as dispersion. Obviously, if you are using a lens to form an image in your camera you want minimal dispersion. Otherwise you will see color fringing around objects in your images. One reason for using multiple elements of different glass types in a lens is to correct for dispersion. It’s relatively easy to design lenses corrected for two colors, and such lenses are called achromatic. Most old quality lenses are achromats. But the ideal is to eliminate all dispersion, or at least as much as possible. Lenses corrected for all visible colors are called apochromatic. Apochromats used to be very difficult and costly to make. This is still somewhat the case, but new glass types called LD, VLD, ULD, etc., for Low Dispersion, Very Low Dispersion, Ultra Low Dispersion, etc. have been developed to help solve this problem, which is worse with long, or telephoto, lenses. Sometime in the 1960s, I believe, it was discovered that natural fluorite crystals exhibited extremely low dispersion, and were ideal for use as lenses. Unfortunately, fluorite is very difficult to grind and polish into lens elements and suffers degradation if exposed to the atmosphere, so must be used only for internal elements in well-sealed lenses. So far as I know, only Canon currently uses fluorite elements in some premium telephoto lenses, made from synthetic fluorite crystals that they grow. Other firms have concentrated on developing glass types that incorporate fluorite or mimic its characteristics. You will often see terms like low dispersion, Ultra-Low Dispersion, ULD, Fluorite Glass, etc., used in lens advertisements. Now you know what they’re talking about.
Another term you will see in lens ads is aspherical, or aspheric. Literally this just means not spherical. As I said in my previous post about optics, most lens elements are spherical; meaning that the surfaces are segments of a sphere. As I said, this is fine if you’re focusing the image on a curved surface like the retina of your eye, but film and digital sensors are flat, not curved. One solution to getting lenses that will project images onto flat surfaces is to use aspheric elements, that is lens elements whose curvature varies from the lens center to the edges. Regular elements are made from lens blanks, wafers cut from cylinders of optical glass. These are ground and polished to the desired curvature by machines that start out with coarse grit and use progressively finer grit until the rouge used for the final polish. But these machines are able to only create spherical surfaces. To make ground and polished aspheric surfaces requires much more complex machinery and processes. Thus, ground and polished aspherical lens elements are costly and so are the lenses incorporating them.
In the mid-80s engineers at Canon developed a process to mold heated optical glass into aspherical lens elements. This was a major breakthrough, but was limited to lens elements of relatively small diameter. I understand that they have now considerably increased the maximum possible diameter. Other firms developed “hybrid aspherics” in which a molded plastic aspheric surface was bonded to a glass element. Some used aspheric elements made completely of molded plastic. If you look at diagrams of complex lenses you will see that two or more lens elements are often combined into one. The separate elements are bonded together with transparent optical cement.
Three people taught me the most, Wolfgang Volrath, Herwig Zorkendorfer, and. Les Stroebel. I never met Stroebel, but he is the author of Applied Photographic Optics, the standard technical book on the subject, a professor at RIT for many years. Wolfgang Volrath was, at the time I knew him, the chief of lens design at Leica. Herwig Zorkendorfer is an old friend who operates a business in Munich making specialized optical gadgets (www.zoerk.com). I’ve used and written about his products many times. Using his adapters you can mount enlarger lenses onto your SLR with both tilt and shift. Enlarger lenses are mostly of very high quality, and with the decline of the darkroom, you can buy even the best cheap. Other of his adapters let me use my collection of Carl Zeiss Jena MC lenses (50 mm, 60 mm, 80mm, 120mm, 180mm, and 300mm, originally for the Pentacon Six/Praktisix/Practica 66 line of cameras) on my Mamiya 645 cameras and on my Canon EOS cameras, the latter with shift and tilt. Herwig is an old hand in the photographic industry, having worked for Heinz Kilfitt in Munich, one of the makers of exceptional quality lenses after WW II (later. bought by Zoomar, for whom Kilfitt built lenses), and Mamiya Germany. I’d ask him a complex optical question over lunch at a street cafe, and he’d proceed to fill napkins with diagrams and equations, usually going far beyond the answer to my question.
Wolfgang Volrath was a different matter. His English is limited, my German is limited, and the translator we had didn’t know any of the technical optical terminology, so we communicated mostly in drawings. I was introduced to Wolfgang by Dennis Laney, my editor at Hove Foto Books and an expert in the history of Leica. Dennis had worked with me on my first book, and all the successors I wrote for Hove. In his book on Leica lenses, Dennis had quoted liberally from Wolfgang, and I could not wait to meet this man who had designed the optics for some of the best lenses ever made. To my surprise, Wolfgang told me that he was a nuclear physicist by training. But, he said, “a ray trace is a ray trace.” Leica, he said allowed him to design the best possible lenses, cost no object. His 100mm macro lens for Leica SLR cameras is without question the best lens of its type ever. It uses one element made of a special glass that Leica makes from scratch in a small laboratory in one of the old buildings in Wetzlar. I was fortunate to see this process on a visit to Leica not long after they had moved production from the old buildings in Wetzlar to their very modern new facility in Solms. But the glass making, at least at that time, was still being done in what looked like a medieval alchemist’s laboratory in Wetzlar. It appeared to be as much an art as science, with the glassmakers putting the raw ingredients into heavy platinum crucibles that were lowered into the furnace. Once the brew had cooked long enough, the crucible was lifted from the furnace, and the molten glass, glowing red-orange, was poured into wooden molds. That’s right, the molds were wood. And they didn’t char or catch fire, I don’t know why. Once the rough block of glass cooled, which was done slowly to avoid internal stress, it was cut into cylinders that were sliced into blanks that were ground and polished into fine lens elements. Only certain special lens elements are made this laborious way. Most elements are made from ordinary crown and flint glass which is sold on the worldwide commodities markets. When I was at Solms they were just quality testing a batch of glass that had come in from Tamron. Those ordinary optical glasses might be bought from any number of suppliers in Europe or Japan (and today probably from China or South Korea). There’s nothing special about them, no matter what you may be told by enthusiastic salesmen.
Modern lenses typically have multiple lens elements, each a discrete lens, and designed to work together to produce a quality image. The main problem with easily manufactured simple lenses is that we want to project the image onto a flat surface, film or electronic sensor, while simple lenses focus their image onto a curved virtual surface. That’s why the retina in your eye is concave. When you project such an image onto a flat surface you will find it impossible to get both the center and outer areas in focus at the same time. Focus on the center and the outer areas are fuzzy, and vice versa. Some portrait lenses are intentionally designed not to correct this and allow a face to be sharp and everything else to be soft. But most of the time that’s not what we want, so lens designers go to great ends to eliminate this so-called “spherical aberration.”. They do this with multiple lens elements, each designed to correct for the problems of others.
Most lens elements are spherical. That means that the curve of the glass is a segment of a sphere. Lens elements can have convex, concave, or plano (flat) surfaces. A plano-convex element would be flat on one side, convex on the other. Similarly, a biconvex element would be convex on both sides, Generally, convex surfaces converge light, whereas concave surfaces diverge light. A common example of a biconvex lens is an ordinary magnifying glass. Often two or more elements are cemented together into a unit, called a group. When you look at ads and product reviews you will see descriptions of lenses saying the lens has “12 elements in four groups” or something like that. A single un-cemented element counts as a group in these schemes. Does this tell you anything important,? Not really. Buying a lens simply on the number of elements/groups is like buying a car based on how many pistons it has. More elements and groups doesn’t make a better lens; some excellent lenses are simple in design.
About The Author: Bob Shell is a professional photographer, author and former editor in chief of Shutterbug Magazine. He is currently serving a 35 year sentence for involuntary manslaughter for the death of Marion Franklin, one of his former models. Shell was recently moved from Pocahontas State Correctional Center, Pocahontas, Virginia to River North Correctional Center 329 Dellbrook Lane Independence, VA 24348. Mr. Shell continues to claim his innocence. He is serving the 11th year of his sentence. To read more letters from prison by Bob Shell, click here: http://tonywardstudio.com/blog/bob-shell-stone-walls-do-not-a-prison-make/