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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