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#21
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Kodak Ektar 127mm lens
"Neil Purling" wrote
The acid test [of my lens] would be a shot of a brick wall, taking care to mark the wall with the aperture in use. I have found a large sheet of newspaper (The Times) makes a very good quick and easy test target. I often cut up a good black and white picture from a glossy magazine and paste the pieces in the center and corners of the frame. Examining the pictures dots under high magnification is a good test of lens resolution. -- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. |
#22
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Kodak Ektar 127mm lens
Most of these sort of lenses are lousy 6 ft or less so the target has to be
something at normal taking distance. I think a wire fence would also work, but a brick wall is usually flat & you can see how sharp the mortar joins are. |
#23
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Kodak Ektar 127mm lens
"Neil Purling" wrote in message ... Most of these sort of lenses are lousy 6 ft or less so the target has to be something at normal taking distance. I think a wire fence would also work, but a brick wall is usually flat & you can see how sharp the mortar joins are. That is quite a statement! I haven't found it true. Kodak Ektars hold their corrections very well down to nearly 1:1. Zeiss Tessars also work well close. Actually, six feet is "normal" distance for general photography. Most general purpose lenses, which are 'optimised' at infinity are actually optimum at something like twenty to fifty times focal length to get a compromise over a wide range of distance. The "optimum" comes from the fact that its not possible to correct a lens for both spherical aberration and coma at more than one distance. Usually, coma is the aberration which becomes apparent at other distances. Since both spherical and coma vary with the stop stopping down will get rid of them in a reasonably well corrected lens. Most of the lenses used on press cameras are very good. The requirement was for very sharp images virtually anywhere in the field and for lack of focus shift, a necessity where rangefinder focusing is used. The poorest performers are the Wollensak Raptar, also sold as the Graflex Optar. These, in my experience, have excessive coma for some reason although they are very sharp in the center of the field and have virtually no focus shift. However, an F/4.5 Ektar or Zeiss Tessar will be free of coma at around f/8 to f/11 where the Raptar/Optar must be stopped down to f/22 or even smaller. -- --- Richard Knoppow Los Angeles, CA, USA |
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