In article <4s1ecg$cq1@nntpb.cb.lucent.com>, rma@clockwise.mh.att.com says...
[......]
>
Why you can't trust lens tests!
>
>Lots of magazines and a few newletters publish the results of lens tests.
>Even some individuals post results of lens tests to the rec.photo newsgroups.
>The big question is just how good are these tests and how much can you depend
>on the numbers they give?
[long, excellent post deleted for space reasons - go back and read it!]
BRAVO!! (Though it is too bad that there is no really good-source/easy-way
to obtain information on lens performance that is relevant for the
particular user..... [though I have posted Nikkor lens evaluation numbers
based on what I consider important aspects of performance].) I started an article last year on a simplified lens checking procedure that has served
me well for comparing lenses, and for finding good/defective lens samples quickly and easily, but I got bogged-down while trying to verbally describe it, alas. The following is an excerpt from that article describing some of
the pitfalls of traditional methods of evaluating lenses:
"Using lens test charts has many pitfalls for establishing lens
quality: the commonly used distance of 25X the focal length may place
many otherwise fine lenses at a distance that is unlike what would be
used in normal photography, and at which the lens performance is not
representative (and may not be very good); it is difficult to test
lenses using test charts at all the relevant distances (especially
near infinity) that would show how a lens performs (lens performance
varies at least some with distance, and may vary a large amount,
depending on the type and design of the lens); it is VERY difficult
to focus with sufficient accuracy on a flat target to make the test
meaningful without using sequence focusing techniques at each aperture
(and a lot of film and eyestrain); it is difficult to align the film
and target planes sufficiently well to make the off center target
information reliable without special alignment tools or great care -
and an otherwise sharp lens may have slight field curvature which
could make the edge test results look poorer than the lens actually
is in normal use; resolution tests may not tell much about contrast,
which in some images may be more important than resolution (both
contribute to the look of sharpness in a lens) - also, in lens design,
it is hard to improve both contrast and resolution together, and much
easier to trade one for the other (it is quite possible for a lens to
be very high in resolving ability, but look poor in picture taking -
and the other way around); it is difficult to duplicate resolution
test results, even when using the same setup, equipment, and
materials - which leads to questions about the reliability of
solid-seeming test result numbers; a meaningfully thorough chart
testing of even one fixed focal length lens (let alone a zoom, or
two lenses tested together for comparison) is an exhausting ordeal
(not a suitable process for anyone but the most dedicated lens tester)
and the resulting sea of numbers may be overwhelming, and may be less
than easy to translate into a good understanding of how one lens
performs compared with another.
On the other side, taking a few photographs to judge the optical
quality of a lens also has pitfalls: what looks sharp enough on a
print (even an 8x10) may not look sharp on a negative examined with
a good magnifier (or on a larger print or projected slide);
cross-lighting on the subject photographed can mask faults that
would be evident in a larger collection of photographs taken under
a variety of lighting conditions; optical faults can easily escape
detection (until after the return period [or warranty period]
has expired - when they have a habit of suddenly making themselves
very obvious); many people seem completely unaware of image quality
anywhere but in the center of the image, but a photograph is
EVERYTHING inside the frame edge, and when the photo viewing (and
taking) sophistication level improves, the "sharp" lens (with the
soft edges and corners) may not look so sharp anymore; one person's
"looks sharp to me" comment may not serve another's desire to know
how good a lens is, since there is no reference standard of quality."
(The above quoted excerpt is Copyrighted 1996, D. Ruether)
Hope This Helps