Media Cybernetics White Paper Spherical Aberration

I n t roduc t ion Digital photomicrographers assume that lens aberrations are corrected by the microscope manufacturer and that their instruments are designed for best performance. However, this belief is unwarranted if the specimen is mounted in a medium inappropriate for the objective lens. It is easy to forget that the refractive medium between specimen and objective is critical for maintaining the microscope’s optical correction. After all, it is a simple air or liquid medium that does not require the careful polishing, crafting, and alignment of an optician. Nonetheless, cavalier treatment in selecting this component can result in a crucial loss of optical performance. A failure to account for the medium’s refractive index or thickness can result in a critical degradation of optical performance by inducing spherical aberration. It should be remembered that the majority of ground lenses cannot focus all the light to a single point. Light rays passing through the periphery of the lens will come to a different focal point than rays passing through its center. (Needham, 1958). This results in blurring of the subject’s edges and the formation of an overall haze that reduces contrast. This is especially evident in fluorescence or darkfield illumination whose black background emphasizes the presence of out-of-focus light. (Figures 1A-B).