
small aperture and high magnification, respectively. These approximations
begin to break down at low power and high NA, which normally do not occur
together. One can compute and compare f and a, or the angles arctan(a=2d
f
)
and arcsine(NA/n) to quantify the degree of approximation.
2.2.1.4 Lens Shape
For a thin, double-convex lens having a diameter that is small compared to its
focal length, the surfaces of the lens must be spherical in order to convert
a diverging spherical entrance wave into a converging spherical exit wave by
the process of diffraction. Furthermore, the focal length, f, of such a lens is given
by the lensmaker’s equation,
1
f
¼ n 1