Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site drusd.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!harvard!talcott!panda!genrad!decvax!harpo!whuxlm!whuxl!houxm!ahuta!drutx!drusd!phl From: phl@drusd.UUCP (LavettePH) Newsgroups: net.rec.photo Subject: Re:re:re: Camera Vibration Message-ID: <1261@drusd.UUCP> Date: Tue, 26-Mar-85 12:16:19 EST Article-I.D.: drusd.1261 Posted: Tue Mar 26 12:16:19 1985 Date-Received: Tue, 2-Apr-85 20:06:12 EST Organization: AT&T Information Systems Laboratories, Denver Lines: 149 >Well, yes, but the performance of these telescopes is limited by diffraction >(if I understand correctly). The analogous situation in photography is that >a lens that is stopped down all the way will produce a somewhat unsharp >image, and that opening it up several stops will produce much better >"brilliance". But continuing to open it up will produce less sharp images >again, due to aberrations in the lens that get worse at larger apertures. >Often, a f/1.4 lens will produce a poorer image than a f/1.7 or f/2 lens >when both are used wide open, in direct contradiction to what you are >trying to tell us. Sure it gets worse. But why? Take a look at the design of the refracting tele- photo lens. Particularly the popular zooms. Count the number of internal lenses. Each element adds reflecting surfaces, abberations, etc., that detract from image sharpness no matter how well the lenses are coated or how well the lens is internally baffeled. Add the problems of positioning internal moving elements and throw in a tele-converter and its a miracle anything in the way of an image makes it to the film. Don't get me wrong here. The modern zoom lens is a marvel of compromise and computer aided design. There is nothing like it when you don't want to lug a lot of lenses around with you. They work great if you don't try to push the zoom ratio too far and realize (as you do) that their best capabil- ities are achieved at around three to four stops back from their maximum open- ing. For long lens telephoto work you need as large a front lens as you can carry and one that is designed to work wide open without a lot of intervening glass to screw up the image. The sharpness of variable aperature lens also falls off at the extremes because of the diaphram,itself. The aperature of the diaphram affects the image quality. Back in antiquity a "good" normal lens worked between f6.3 and f128. Later on the range became between f3.5 and f22. Lately, the range is between f1.2 and f16. The manufacturers of variable aperature lenses realize this limitation and design the lens to work best at some median point with equal degradation of the image at the extremes. For optimum sharpness you need a lens that is *designed* to work best at its maximum aperature. (Or use the variable lens only at its best aperature if you have no alternative.) This all started over sharpness problems in telephoto shots. Who but a masochist or astronomer would attempt long lens work at f2? Think about it: APPROXIMATE DEPTH OF FOCUS AT f2 WITH LENS FOCUSED AT 100 YARDS FOCAL LENGTH NEAR POINT FAR POINT DEPTH OF FOCUS 150mm 167 ft 1500 ft - 300mm 250 ft 375 ft 125 ft 500mm 280 ft 323 ft 43 ft 1000mm 295 ft 306 ft 11 ft 2000mm 299 ft 301 ft 2 ft APPROXIMATE DEPTH OF FOCUS AT f11 WITH LENS FOCUSED AT 100 YARDS FOCAL LENGTH NEAR POINT FAR POINT DEPTH OF FOCUS 150mm 55 ft inf - 300mm 143 ft 1nf - 500mm 215 ft 496 ft 281 ft 1000mm 273 ft 333 ft 60 ft 2000mm 293 ft 308 ft 15 ft I've used a rough approximation here by assuming a 0.004" circle of confusion in determining the hyperfocal distance. If you halve that circle diameter you drastically reduce the depth of focus. (e.g. For the 150mm lens the NP becomes 254 ft, the FP becomes 367 ft and the depth of focus becomes 113 ft at f11.) Cut your point of focus down to fifty or a hundred feet and your depth of focus becomes so shallow at f2 that you have to ask that pretty snake to hold still while you measure the distance to his fangs and rattles with a steel tape. :-) A 1000mm, f2, cat might be nice for low light work but it would be a little hard to carry around. I think a lot of problems thought to be shortcomings in camera or lens design could simply be the photographer's inability to focus precisely. Could it be that a hunter's optical rangefinder might be a better investment than a more perfect lens? >>There is no difference between telescope and photographic lenses except for >>the outward appearance. >Not really true. How close does your telescope focus? How light is it? >These are important in photographic lenses. So is cost. The only difference between the Celestron C-90 spotting scope and telephoto lens is the barrel of the scope is painted orange and the telephoto is black. They both focus to ten feet, they have identical Maksutov-Cassegrain optics. Mine weighs in at about 3 pounds. The lens costs somewhere around $300-$450 depending what goodies you buy along with it. It has a 1000mm focal length, a 90mm objective diameter and a focal ratio of f11. My Celestron C-8 focuses down to 25 feet, weighs 21 pounds and cost about $1200. It has a 2000mm focal length, a 208mm objective diameter and a focal ratio of f10. The Schmidt-Cassegrain optical design is quite similar to the flat fronted mirror lenses used in general photography. Many serious amateur astronomers spend more time photographing than star-gazing. They have to with exposure times measured in fractions of an hour. :-) >Telescopes and photographic lenses obey the same optical laws, but different >tradeoffs are taken in their design, producing final results whicy may >differ. No argument here. The problem is that the photographer must trade off resolution and contrast for portability. Unfortunately, those are essential elements of any truly sharp photograph. If you want the details of a spider spinning its web at 30 feet, the whiskers on a muskrat at 50 yards or the limb and branch detail of a tree at a half-mile you have to be willing to give up some of the portability. There is a good example of photographic sharpness and contrast for various tele- photo lens diameters in the current Celestron catalog. Meade and Questar probab- ly have them, too. Look for the moon photographs taken with the various diameter lenses at the same effective focal lengths. Look particularly at the small cra- ter detail and the overall contrast (the brilliance). The only difference be- tween photographing the moon and a terrestrial subject is the amount of atmos- pheric distortion between the photographer and the subject being photographed. The same rules of visible light optics that yield the small craters around Tycho also get you the wing detail on a dragon fly on the other side of the pond or the rigging on the sailboat far out on that lake. (A photograph of a quarter, half or three-quarter moon makes a good test of any telephoto lens 300mm or above, by the way. Some of the craters should begin to appear on your negative.) I sometimes go picture taking in the outback with a friend. We both use Pentax bodies. He uses the 1000 ASA print film, a rather expensive zoom lens and an equally expensive 2X extender that brings his effective focal length up to about 280mm. He uses no filters or lens hood. When I don't feel like lugging the 1000mm big gun around I usually use one of Joe Spira's 300mm mirrors with a pol- arizing filter (also a great substitute for a ND filter) and a hood. I usually load 400 ASA film. He is endlessly bewildered by my getting prints that are us- ually both sharper and have more "snap" than his with a lens that cost less than his extender. The only difference he can see is that I usually brace my camera- hand against a rock or tree while he just holds his up to his eye and shoots. There are many factors at work here. Aperature is only one of them. My advice remains the same. That is, if you want really sharp telephoto shots: 1. Support the camera on a solid base. 2. Get the mirror vibrations settled down before you release the shutter or damp them out as much as you can. 3. Get as much useable glass up front as you and your wallet can carry. 4. Avoid high speed/low contrast film. 5. Avoid as many unnecessary lenses and filters between your main lens and film as you can. Find something creative to do with those little filters that are intended to screw into the back of the cats. They are nothing but trouble on a dusty day. 6. Shade your lens. 7. The longer the effective focal length, the tougher it becomes to focus accurately. Your split image range-finder becomes useless. Make sure the focus on the ground glass corresponds to the focus on the film plane. 8. Make sure you know what your problem is before you try to solve it with a Nikon or Questar lens. Sometimes expectations just aren't realistic. - Phil