Dear Dave,

Good question, especially as early MRC 500 and  600 scopes (as well
as others) almost never filled the BFP.

Not filling the BFP means that no light goes through the high-NA part
of the lens. Hence, it operates as though it was of lower NA, the
spot is bigger than it should be and the peak power is less.

However, the "good news" is that low-NA lenses are much less subject
to the effects of spherical aberration.  So the bad performance you
get from having a big spot is somewhat offset by the fact that this
performance doesn't get worse quite so fast as you focus down into a
specimen having the wrong refractive index.

However, because the emitted light goes in all directions, the
collection light path does operate at the full NA of the objective.
For this reason, you may not immediately notice either of these
effects of not filling the BFP. Nonetheless, the resolution will be
less and, given a reasonably small pinhole, so will the signal level.

This, in a nutshell, is the reason that the early Bio-Rads worked
better with a 100x NA 1.4 than with the 60x NA 1.4 . The former lens
has a smaller BFP.

Jim P.



>Dear users- What is the effect of not filling the back focal aperture
>of an objective in a point scanning confocal system.  I am assuming
>the same laser power going in, but the beam not expanded as far.  It
>seems to me that it is a different issue than in a normal imaging
>sytem. Would you get a smaller spot of illumination?  Thanks- Dave
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