"Al Denelsbeck" <news@wadingin.net> wrote in message news:sJ8Ma.175912$cm4.4368868@twister.tampabay.rr.com...

> Olaf Ulrich <olaf.ulrich@onlinehome.de> wrote in message

> news:1056987898.406376@nbgm66x...

> > Okay. Asymmetric lenses (and virtually all SLR lenses

> > are asymmetrical) are optimized for some particular

> > magnification, or image-to-subject ratio, or distance.

> > In non-macro lenses, this distance usually is infinity

> > or near-infinity. At any other distance, the lens' per-

> > formance will decrease---the greater the difference

> > to the optimal distance, the greater the loss.

[...]

This is quite correct. I have checked many a lens (see

www.David-Ruether-Photography.com/slemn.html for some...), and

most lenses show considerable sharpness variation with

focus distance even within their "normal" focus ranges - and

zooms often do not perform very well near their close-focus

limit (and tubes degrade this performance considerably more

by pushing the lens well out of its design range). With zooms,

achromats generally give sharper results than tubes do...