I'm surprised Nick White hasn't contributed to this one, but since\
he hasn't I'll chip in.  As he regularly points out to me, the
main cause of falling-off of image quality as you go in deep is
spherical aberration caused by poor index matching.  If you are
using an oil-immersion lens and your sample is mounted in a medium
of refractive index 1.5 (1.515 for pedants) this will not happen,
and the higher NA you have the better (except when your lens hits
the coverslip).
 
If you are using a dry lens you will only have optimal imaging
(whatever your mounting medium) at one distance.  This corresponds
to straight beneath a .17mm thick (#1.5) coverslip.  However, if
your lens has a correction collar for different thicknesses of
coverslip you can tune this to different depths.  This can be a
great improvement.
 
If your specimen is in water you are best off with a water-immersion
lens.  Sure, the NA written on the barrel is lower than on your oil
lens, BUT THE NA WITH WHICH YOU ARE IMAGING YOUR SAMPLE IS NOT!!
If your sample is in water your lens cannot have an NA of greater
than 1.2 whatever it has written on it.  Your 1.4 NA plan-apo will
actually have an NA of about 1.1 when imaging a sample in water.
 
Remember that the depth resolution depends on the SQUARE of the NA,
so bigger is very much better for confocal imaging.  But this only
applies if your refractive indices match.  If they don't, bigger is
not only not better, it isn't even bigger!
 
                        Guy Cox
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