Piston, D. W.
Choosing objective lenses: The importance of numerical aperture and magnification in digital optical microscopy.  Biological Bulletin 195: 1-4 (1998).

Microscopic images are characterized by a number of microscope-specific parameters-numerical aperture (NA), magnification (M), and resolution (R)--and by parameters that also depend ont eh specimen--for example, contrast, signal-to-noise ratio, dynamic range, and integration time. In this article, issues associated with the microscope-specific parameters NA, M, and R are discussed with respect to both widefield and laser scanning confocal microscopies. Although most of the discussion points apply to optical microscopy in general, the main application considered is fluorescence microscopy