On 4 Aug 2002 06:32:21 -0700, jchapman11@cox.net (john chapman) wrote:

>Thanks for your comments.
>
>This was the first time I had scanned film strips (normally only scan
>slides) and I was having problems with the film strip holder. As to
>the scratches...
>I test all my lenses. I had never intended to publish the results
>anywhere, so once I had satisfied myself as to the results, I was not
>particularly careful how the film was handled and stored.
>
>The Nikon 400/3.5 is impressive optically, and provides a great max
>aperture for high shutter speed and short DOF. However, the Sigma
>50-500 has a street price something like 1/4 that of the Nikon, has an
>incredible zoom range, provides very good optical performance that
>seems to be second only to the Nikon, and has HSM focusing. There
>will be times when the lens speed of the 400/3.5 will be required.
>But for most shooting, I think the flexibility and focusing speed of
>the S50-500 is a better choice.
>
>I think the 500mm test with this S50-500 shows pretty good corner
>resolution. I suspect the corner resolution of some of the tests is
>slightly less than what the lenses are capable since focusing at
>something near 5.6 will not be as accurate as focusing at 3.5 . I
>think both I and the in-focus indicator in the camera do better with
>the brighter, shorter DOF images provided by faster lenses such as the
>Nikon 400/3.5 .
>
>Anyhow, the web site was hit something like 39 times in the first 12
>hours it was up. I am pleased with the interest, and hope it is
>helpful for some people. I think I will do some additional tests on
>the S50-500 using ISO100 and 400 slide film. ISO100 will show what
>the lens is capable of, while the 400 will show results using the film
>speed most likely to be used with the lens. Probably, I will include
>images at other focal lengths as well.

Problems I saw in the tests: lens performance is not
necessarily the same at near and far focus distances
(it is often not, actually...), so checking resolution
at only one (close) distance may not be a good indicator
of how the lens performs for most uses; it is difficult
to get the test-surface plane exactly parallel with the
film, giving erroneous results for at least part of the
frame (and lenses often *do* have tipped planes of focus,
a useful thing to check for); it is VERY difficult to
focus accurately on a planar subject, and *any* error
will degrade the indicated performance in the test; more
true for wide angles than teles, but B&W tests will often
show different results from color tests in the image
corners; a good test indicates the performance level
in the far corners, at the far edges, possibly at the
near edges, and in the center (it is likely that all
three [four] locations will show different results - but
if one is lucky, the four corners will be the same, as
will be the two far edges [and two near edges]). For
www.David-Ruether-Photography.com/slemn.html, I use comparative
subjective testing, using infinity-focus large-area
non-planar subjects (except for the close-focus checks)
to reduce some of the problems noted above (it helps
to have available a good, wide-angle view of a
populated hillside top use as a "target"...;-).