Richard Levenson asked about high NA lenses:
 
A correctly used high NA lens will never be out-performed (by any criteria
except for working distance) by a lower NA lens.  However, the problem is
often finding the lens which is correct to use for a given application.  In
an upright microscope, one can often use water immersion lenses without
coverslips to attain excellent light collection and penetration depth.  In
an inverted microscope, though, your pretty much stuck with a coverslip and
that's where the problems start.  Lenses are corrected for imaging directly
inside the cover slip.  As one focusses deeper into the sample, homogeneous
media (air or immersion fluid) is beeing replaced by biological stuff. This
is not covered in optical engineering courses, nor is it the same for any
two imaging situations.  Therefore a lower NA lens, which has less
complicated optical corrections, may show more consistent behavior as a
function of the focus depth.  If this doesn't make sense to people, I can
try to give some examples . . .
 
As far as Nikon lenses go, I believe that they are the only NEW lenses
available that are designed for 160 mm tube length instead of infinity
corrected, so they would, of course, be the best for a Nikon system.  (You
can use 160 mm lenses with infinity corrected systems and vice versa, but
the depth variation problem I mentioned above gets even worse).  One
solution is to try to find old lenses that dealers have had sitting on
their shelves.  I've found these lenses as good or better than many new
lenses.  There are people who'll help you find these gems, look in the back
of microscopy trade journals (No advertising on the net, remember).
 
I might also add that not all lenses are created equal: of 5 40X 1.3NA oil
imm. lenses that I tried (all from the same anonymous manufacturer), 3
performed well and 2 showed lower light throughput, and worse chromatic
aberrations.  Of the three decent performers, one yielded the best
throughput versus focusing depth by about 50%!  I will leave as a homework
exercise for you to determine which one I kept.
 
                                Dave Piston
                                Vanderbilt University