In article <96048.103935U15310@uicvm.uic.edu>, U15310@uicvm.uic.edu says...

>When using an f/2.8 lens without a converter, it becomes sharpest in >the range of f/8.When using with a converter, is this still the case, >i.e., stop the lens down to f/8 giving an effective opening of f/16?
>Or, in the case of wanting maximum sharpness and dept of field
>without the converter I might stop down to f/11. Does this mean I >would need to do the same with the converter, giving an effective >opening of f/22?

Alas, performance may not peak with the 2x converter until f11, or
even smaller with a lens that doesn't work well with converters - resulting in mighty small effective apertures to deal with! (There
are some combinations that are better, like the 300mm f2.8 EDIF MF Nikkor with the TC301/300, which is useable wide open, and is quite
good by f5.6 [f11]). Performance quality of a good 2x converter on a particular lens is hard to predict, though, in general, I find that
the "short" versions of 2x's are best on 28-105mm prime lenses, and
acceptable-to-good at smaller apertures on longer lenses and on only
a few zooms. The "longer" versions of 2x's work well on the big, expensive speed teles - and acceptably on some slower ones, well on others. Maybe the best use for the "short" 2x converters is in macro work at small apertures. Maybe 1.4x lenses are more useful, since
they work well with more lenses, and at wider apertures (and the
speed loss is only one stop). (But, by continuing this logic, we
come to the conclusion that 1x converters are the most useful
of all, since they introduce no optical problems, and
lose no speed ;-)
Hope This Helps