To achieve high resolution in the z axis, you may want to do:
 
(1) use high NA objective.
(2) minimize spherical abberation of the objective by matching the
refractive index of the specimen to the value which objective was designed for.
For example, use water immersion lens for water based sample, use xylene or
methyl salicylate (n=1.52) to "clear" your specimen when you use oil immersion
lens.
(3) try not to obtain optical sections too deep within the specimen.
 
 
P. C. Cheng
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