In article
[most of an interesting post deleted...]
>..................................................... Another even more
>important point that's often overlooked: "Tests" based on ANY kind of
>flat-chart measurement are almost totally irrelevant to normal photography
>of 3-dimensional objects, and are almost completely useless for predicting
>*real-world* lens performance. Quite a few years ago I read an article in
>"Pop Photo" quoting the optical director of the Carl Zeiss Foundation as
>saying that it's impossible *in principle* to arrive at a single "figure of
>merit" that will quantify photographic lens performance, and I've always
>tended to accept that as the final word.
I agree with you, and I check lenses using "real-world" subjects. BTW,
for a post that never materialised (describing the fairly easy process
I do use for testing has escaped me so far, though I may have accomplished that in my post describing the performance of the Nikkor 24-120mm zoom),
I wrote this about chart testing disadvantages:
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) [...]; 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.
Hope This Helps