Archived posting to the Leica Users Group, 1997/12/18
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Jim Brick wrote: >At 11:59 PM 12/17/97 -0500, you wrote: >>At 12:06 AM 12/17/97 -0800, you wrote: >> >>Jim: >> >>So perhaps my practice is just making the most of DOF. Thanks. >> >>Chuck Westphal of Canon explained to me that longer lenses with flourite >>and other rare earth glass are made to focus past infinity because the >>glass can expand in very warm situations, shifting the focus points. I'm >>not sure I buy it. >> >>Tom > >Tom, > >I personally am skeptical about a lens changing focus points based on (snip) Chuck Westphal is right. All lenses exhibit some focal length changes with temperature, but it's only noticeable in longer lenses, as DOF easily covers these changes in shorter lengths. A bit of reflections will make you realize this. All materials exhibit a change in size with temperature. Most expand with rising temperature, some contract. The glasses in a lens will show variations between them, and the rates of expansion or contraction will also be different from those of the lens mount materials. If you have ever used a mirror lens, or a reflector telescope, you will see this easily. These types of optics usually show a marked change in focus point, unless specially corrected. I've had a flourite lens, and have used a couple of others that definitely showed that the infinity position (and all other distances) had to be refocused with varying temperatures. I now have 180, 300 and 400mm Nikkors with ED glass in them (actually, I believe the 400 may have a flourite element, but I haven't been able to confirm this) that all exhibit this behaviour, but to a lesser degree. So Jim, if you have a remote setup that you prefocussed at freezing temperatures, and are expecting to get a sharp picture after the sun hits the lens, good luck! Unless the temperature/focus point curve puts these two conditions on the same focus point, you won't get the sharp picture you expect. That's a fact of life (and optics) that nobody has yet found a solution for for general camera optics. It's akin to demanding that all colors focus on the same plane, period. It just doesn't happen. BTW, on this 'mushy wide angle' thing. Wideangle lenses _are_ sharp. This can be proven easily by almost any type of objective test. The only problem is that when we compare pictures taken with a 50 with those taken with a 21, we compare the same objects in a scene, and we expect to see the same details with the same degree of sharpness. Intellectually we know that the objects in the picture taken with the 21 are only 42 percent as large, linearly, as the same objects taken with the 50, but the intuitive part of the brain doesn't get the message, like it would in the real world, where depth perception and understanding puts the smaller objects futher away. Here the objects in the picture are the same distance away (slides on a light table), so we expect the same amount of detail. This is a psychological thing, not an optical problem. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com