The Plan Fluor 40X NA 1.30 oil immersion lens combines magnification, high NA, field flatness, long working distance, and high ultraviolet transmission optimized for efficient work in fluorescence studies. For lenses of equal magnification, image brightness in epi-illumination systems is proportional to the fourth power of the numerical aperture. At this degree of magnification, an objective needs a fairly high NA to provide the richness of detail that 40X magnification should reveal. The lowest NA usually provided for 40X lenses is 0.65. Even at this NA, the optical performance of the microscope becomes dependent on the one optical component the lens designer and manufacturer cannot control - the coverslip on top of the specimen.
Many specimens are prepared with coverslips of other thicknesses.
This is of little consequence for observations at low powers, but it has a strong influence at 40X and above. While 0.65 is the NA of ordinary 40X lenses, the higher-performance dry objectives can be as high as 0.95. These are so adversely influenced by coverslip thickness that a correction collar must be fitted on the objective. Image quality is optimized by turning it while observing image appearance. Turning the collar alters the internal spacing of some components of the objective, which, unfortunately, also changes the effective focus of the lens.