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. In practice, one must simultaneously turn the collar with one hand and the fine focus knob with the other, while looking for the crispest image. Many users never master this technique. For NAs higher than 0.95, immersion lenses are offered. Every manufacturer lists different special objectives for observation techniques other than ordinary brightfield - techniques such as phase contrast, DIC, and fluorescence.