Hi-- It is too early in the morning - NOT my best time, but I am up
to deliver some slides I shot last night of 12 story high "96" lights
on the Ithaca College towers. It was CCCCCCOOOOOOOOOLLLLLLLDDDDDDD!!!

>Ok, now I know what you might be talking about. Here is a long shot:
>
>The problem something like this is discussed in Goldberg's book
>"Camera Technology: the Dark Side of the Lens".
>
>He claims that most lenses have a certain amount of curvature of
>field. If the user focuses right on at the center, subjects will be
>off at the edges. (This must be assuming distant or flat subjects.)
>To get better average perforamance some manufacturers actually make
>the focusing off a bit (based on the average curvature in their lens
>lineup) so that when the user focuses in the center, a band part way
>out will be sharp. He suggests that an intentional focus offset of 50
>microns is not uncommon. The thing that they make deliberately off
>could be just the split image.

Few Nikkors have detectable field curvature, but a few do (55mm f3.5,
135mm f2, 15mm f5.6, one 16mm f3.5). Being reflex-focused, I think
there is no way to build in a compromise focus ability (if I am right
[and I am only guessing....], the edges are what focus is based on with
AF and the split image prism, but the center [or wherever you choose to
focus] is what is sharp with ground glass focus. In RF or twin-lens
cameras, a compromise is possible, since the taking lens is not used for
focus (though my Rollei twin, with the f3.5 Zeiss Planar [which has very
noticeable field curvature] makes no attempt to average the field [others
I have tried or owned were similar - ah, those "great" German lenses...]).

>Actually, by now I've gotten confused. I think all we actually know
>about this guy's problem is (1) that the focus shifts as he zooms (no
>big deal), and (2) he says that "the focus is off". Many times
>beginners will say the "focus is off" and all they mean is that the
>index mark doesn't line up with infinity when the focus on a distant
>object. I got a profuse thank you note back from him and he said he'd
>do all the tests I suggested. That will take him a week. :-). We
>will see what he says then. Another possibility is that when the AF
>stops it isn't in focus. If good lenses do it right, that probably
>means the the Sigma has bad mechanical stuff and it coasts or jerks or
>something, so the AF can't get it to stop where it wants it to.
>That's why I suggested that he do AF-assisted manual focusing and
>report to us on that.

All true - I was making assumptions, and tossing out a possible
explanation - based on what he said. Hey, are you turning practical
(rather than theoretical) on us, and am I doing the reverse?!

> -- David Jacobson
>
David