Optical Aberrations and Different Types of Objectives

The two major distortions or aberrations (Davidson and Abramowitz, 1999) in optical microscopy are (i) chromatic aberration which is due to the different d = 1.22(1/ 2NA)

refractive indices of the glass optical element (such as a lens) for different wavelengths and (ii) geometrical or spherical aberrations due to the shape of the lens, in which the rays from the edges of the lens don't get focused at the same point where the axial rays focus. Both these aberrations can be corrected by using lens doublets consisting of two lenses made of materials of different refractive indices. Good-quality objectives may contain multiple lenses to compensate for these errors. These corrected objectives are named achromatic and aspheric objectives. Another aberration in optical microscopy is due to the field curvature (curved image plane) of the objective lens that produces a curved image. New objectives made of special fluorite glass are available which correct for most of these aberrations. Thus one can choose from different types of objective lenses such as achromat, Plan-achromat, Plan-apochromat, PlanFluor, and so on, depending on the application and degree of aberration correction needed.

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