 The numerical aperture of a lens is defined as

where α0 is half the angle spanned by the objective lens seen from the sample, and "n" is the refractive index of the medium between the lens and specimen (≈1 for air).State of the art objectives can have a numerical aperture of up to 0.95.Because sin α0 is always less than or equal to unity (the number "1"), the numerical aperture can never be greater than unity for an objective lens in air.