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re: [OM] FYI - all the 35 mm lenses tested by photodo, in order by Grad

Subject: re: [OM] FYI - all the 35 mm lenses tested by photodo, in order by Grade (m
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Sun, 16 Dec 2001 16:24:48 +0000
At 17:20 12/16/01, Tom T. wrote:
I doubt he tests at infinity. It's hard to get a test chart big enough.

And don't blame him for ignoring stuff. Of course there are other factors
influencing choice of lens. Now there's room for you to tell us about them.

Tom

Some of the details on his site state they're tested at infinity. I doubt he's using film, but that the tests are aerial. Two reasons for this:

(1) To test at infinity focus on film in a camera body, a lens extension would have to be inserted between the lens and the body. While possible, I'm not certain it's that practical.

(2) Along with lens MTF, there is film MTF. If he were using film in a camera, the MTF results would be for the entire optical system, lens and film combined, and the film MTF would confound the results. To isolate lens MTF, an aerial system is used that examines the virtual image behind the lens in space. In this case, the lens alone would be mounted with the aerial measuring system behind it. The distances between chart and lens, and between lens and the aerial measuring equipment are adjusted so that the chart is properly magnified to "full frame" (determined by focal length) and that it's in critical focus at the measuring equipment. Other equipment in an optical lab can determine exact location of front and rear lens nodes, and its focal length. From these, distances between everything can be set precisely. This allows measuring the MTF at the lens infinity focus setting inside the laboratory.

For prime lenses that move the entire lens cell as a fixed unit using a helical, it doesn't much matter what the focus distance is. Examples are the 50mm Zuiko's. For lenses that use "internal" focusing (zooms) or have a "floating element" for aberration correction related to critical focus distance (wide angle lenses), the critical focus distance set on the lens can make a difference in MTF. For these lenses, what distance should be used for critical focus and can it be the same distance for all lenses (some focus closer than others)? Using infinity may be arbitrary, but it's a distance that can be set on all the lenses (OK, very, very nearly all lenses).

-- John


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