A microscope objective with (small) central obstruction of aperture and small wave-front deformation has a loss of amplitude at the center of the Airy disk which may be expressed in terms of losses due to spherical aberration, coma, and central obstruction. These losses are essentially independent; the net loss is the sum of the separate losses. Data are presented from which these losses for Schwarzschild spherical mirror systems of high initial magnification may be computed. It is shown that for visible or ultraviolet light and for numerical apertures greater than about 0.5, the net loss of central amplitude becomes excessive unless the object field is restricted to an extraordinarily small diameter. The numerical aperture at which such restriction of the field is necessary may be increased if the initial magnification is made small, e.g., about 8×, as is possible and convenient for infrared microspectroscopy.
DAVID S. GREY, "Computed Aberrations of Spherical Schwarzschild Reflecting Microscope Objectives," J. Opt. Soc. Am. 41, 183-192 (1951)