At 20:12 3/29/02, you wrote:
All 50mm non macro I have seen has barrel distortion, three 50/1.4 SC and
MC, 50/1.8 MC. 50/1.2, 55/1.2. May be I'm a bit exaggerated about the
distortion, but all the reports I have seen also shown about 1 0istortion.
For normal shooting it is fine, but for copy works, you need a very low
distortion lens unless you don't care about the quality.
C.H.Ling
Based on several lens tests I've read on the "standards," I believe whether
there is *detectable* barrel or pincushion (using specific lab testing
methods with test targets) varies from lens to lens. IOW, manufacturing
variations create some with slight barrel, some with slight pincushion and
some with "none detectable." Whether the "slight" detectable in a lab is
"acceptable" in practical application matters much more versus what's
detectable using a lab test target and highly specialized testing methods
that are designed to specifically detect it and measure its severity.
This should not imply the user "don't care about quality." If it's not
detectable in *practical* use, it's as if the aberration doesn't exist. If
you're seeking absolute perfection in actual lenses, you won't find it. I
guarantee every one of your lenses has barrel or pincushion distortion,
including the 50mm Macros. Non-debatable *fact*. Building a lens with
having absolutely, exactly, "zero" or "none" is like trying to balance a
needle on its point. It has not, cannot now, and never will
happen. Variation in manufacturing guarantees it.
What might also be a fact is it's below detection threshold, even on an
optical lab bench, which brings it back around to the need for it to be
detectable in practical application. Paintings with strong, exceptionally
precise rectilinear lines are very, very rare. I would look at the frame
and its straight inside edges around the painting to see if there's
anything detectable in the image by a human using eyesight alone without
any aids.
If this sounds strident, it admittedly is. While it's worthwhile to ask
questions about aberrations and distortions detectable in a lab, the most
important question is whether the magnitude of it is detectable in
practical use, making practical photographs.
-- John
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