In article <49i354$p5k@cello.hpl.hp.com>, jacobson@cello.hpl.hp.com says...
>In article <49gjjq$2ej@newsstand.cit.cornell.edu>,
>Bob Neuman wrote:
>[ delete stuff about a Sigma lens that allows focusing beyond infinty ]
>>If the lens had "ED" or similar elements, the lens would normally allow
>>focus beyond infinity to allow for changes in focal-length with
>>temperature. At $150, this is unlikely, and may be simple misadjustment,
>>or it could be intended to cover shifts in focus with zooming (not
>>uncommon, even in expensive zooms - and why one should NOT focus at
>>the long end of the zoom range and then zoom back to shoot [as many
>>instruction books used to advise doing]).

>Actually, they still do. Just 3 months ago (late August) I bought a
>Nikon 28-85 f/3.5 -4.5 zoom and the instruction book recommends
>focusing at the long end and then zooming back to shoot. (Arguably
>this lens has been out for a long time and Nikon may have never
>updated the instruction book.)
>What is more interesting is the issue of focus shift. Several friends
>of mine also have the lens. At least one of those lenses has very
>noticible focus shift through the zoom range. Mine does not seem to
>shift. I wonder if Nikon made some subtle changes to the lens
>formula?

I should have left out the words "used to" - maybe it was wishful
thinking, figuring that this bit of bad advice would have dissapeared
by now (guess not!). Zoom lenses vary in whether or not they vary
(noticeably) in focus with zooming both with designs and with particular
manufacturing samples (the 28-85mm has not changed in design). If a
lens shows shift, and if the shift is smooth, and if the greatest error
occurs at the short end, and if the zoom focusses a hair beyond infinity
at the long end (conditions that are met in a good proportion of zooms
showing error, surprisingly), paper or acetate shims installed under
the bayonette mount can sometimes correct some or almost all of the
error. BUT, since there is some residual error in virtually all zooms
(which may or may not be covered by DOF), it is still reasonable to
ignore the instruction to focus at the long end, then zoom to shoot.
Hope this helps.