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Formula Numerical Aperture (Definition) Numerical aperture    Refractive Index    Angle

Formula
Formula: Numerical Aperture (Definition)
Minimum distance between two still distinguishable objects (Rayleigh criterion)

Numerical aperture

Unit
Numerical aperture describes the medium between the enlarger (telescope, microscope) and the enlarger itself.

Refractive Index

Unit
Refractive index of the medium (e.g. air) between the viewed objects and the objective of the enlarger.

Angle

Unit
Half the aperture angle of the objective. The highest aperture half-angle is theoretically 90 degrees.