An idealized thin lens, implemented as a Zernike defocus term.
nwaves : float
The number of waves of defocus, peak to valley. May be positive or negative.
This is applied as a normalization over an area defined by the circumscribing circle
of the input wavefront. That is, there will be nwaves defocus peak-to-valley
over the region of the pupil that has nonzero input intensity.
reference_wavelength : float
Wavelength, in meters, at which that number of waves of defocus is specified.