For a lens the resolving power depends upon the wavelength of light being used and inversely upon the numerical aperture. The N.A. Is the product of the refractive index of the medium (1 for air, 1.5 for immersion oil) and the sine of the angle, i, the semi angle of the cone formed by joining objects to the perimeter of the lens. The larger the value of N.A., the better the resolving power of the lens, most objectives have their N.A. Value engraved on the barrel and this should be quoted when describing an optical system.