Fig. 2.

Numerical study of different approximations of the defocus function for NA 0.3 objectives with defocus distance 65 $\mu \mathrm{m}$ (a), NA 0.6 objectives with defocus distance 30 $\mu \mathrm{m}$ (b) and NA 1.4 objectives with defocus distance 5 $\mu \mathrm{m}$ (c). Theory-Nonparaxial and Theory-Paraxial indicates the difference between nonparaxial and paraxial approximation methods to theory, respectively.