Hi,
I just discovered something I am pretty sure is a bug. Although I tried to dig deep in the source code, I just cannot see where it is coming from. Try the following:

import poppy
import astropy.units as u

wf = poppy.FresnelWavefront(beam_radius=1.0*u.m,
                            wavelength=1.0*u.m,
                            units=u.m,
                            npix=128,
                            oversample=1)
wf *= poppy.QuadraticLens(f_lens=-10*u.m)

and inspect the value of the radius of curvature. I did this by printing to the console the property z in the QuadPhase get_phasor function:

def get_phasor(self, wave):
        """ return complex phasor for the quadratic phase

        Parameters
        ----------
        wave : obj
            a Fresnel Wavefront object
        """
        print(self.z)
       ............

In the above code, I get printed out -10 m (instead of a positive value). Now change the units for the wavelength to something different, e.g. mm or so when initializing the wavefront. The output reads 986960.4401010844 m.

I tried to look at what is happening but I cannot figure it out.