Posted by
Stanislav Vitha-2 on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Condenser-lens-choice-for-a-given-objective-tp7584901p7584926.html
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My understanding is that in brightfield mode with axial light (condenser NA
= 0), resolution d=lambda/NA
With strictly oblique illumination where the angle of illumination equals the
acceptance angle of the objective (0th order light travels through one edge
of the lens, while the 1st order light travels through the opposite edge), the
resolution is doubled: d=0.5*lambda/NA.
With Kohler illumination, i.e. illuminating with a solid cone of light at a
variety of angles, where the condenser NA = objective NA, the resolution is
somewhere in between:
0.5*lambda /NA < d < lambda/NA
So for this setup, the Rayleigh formula (0.61*lambda/NA) is actually closer
to reality than the Abbe formula (0.5*lambda/NA), in my opinion.
For a standard brightfield setup, lateral resolution depends on the total NA
of the system, i.e. the average of the objective NA and the condenser NA,
where the effective condenser NA is equal or less than the objective NA.
This is what Guy was indicating in his earlier post, I think.
Since the effective NA of illumination is only as large as the NA of the
objective, increasing the condenser NA beyond the NA of the objective
(e.g., using a 20x/0.5 objective, condenser aperture opened to NA=0.9) is
not going to increase resolution.
Source:
R. Wayne: Light and Video Microscopy. Academic Press, New York, 2009.
ISBN 978-0-12-374234-6
Stan Vitha
Texas A&M University
Microscopy and Imaging Center