Photo: Tony Ward, Copyright 2018
Bob Shell: Letters From Prison #28
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Letters by Bob Shell, Copyright 2018
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PHOTOGRAPHY & OPTICS
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I promised to talk a bit more about 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. Why this is done will have to wait for another post.
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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/
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