APO
APO is a shorthand for apochromatic. This term is used, for instance, to characterize lenses that minimize chromatic aberration by the utilization of super-low dispersion (SLD) lens elements. Achromatic lenses focus red and blue wavelengths in the same plane. Apochromatic lenses focus red, green, and blue in the same plane. [1] Residual color inaccuracy (secondary spectrum) can be an order of magnitude lower than for an achromatic lens of similar aperture and focal length. Apochromats correct spherical aberration at two wavelengths, not one.
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