Numerical Aperture(NA) NA=1-theoretical Aperture NA=n(sin a) maximum numerical aperture of a lens n: refractive index of the operating with air as imaging medium between the imaging medium the front lens of objective and specimen cover glass Objective lens A Angular aperture Light (≤72 degrees Cone a One half of A-A Specimen cover glass— na of an objective is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance https://en.wikipedia.org/wiki/angular_aperture http://micro.magnetfsuedu/primer/java/microscopy/immersion/index.htmlNA = n(sin ) n: refractive index of the imaging medium between the front lens of objective and specimen cover glass Numerical Aperture (NA)  Angular aperture One half of A-A NA=1 - theoretical maximum numerical aperture of a lens operating with air as the imaging medium (72 degrees) https://en.wikipedia.org/wiki/Angular_aperture http://micro.magnet.fsu.edu/primer/java/microscopy/immersion/index.html NA of an objective is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance. Objective lens Specimen cover glass