Robert Moerland

Paraxial focal length: fix wrong sign for negative lenses

3

# Why The paraxial back focal length `f2` of a negative lens is returned as a positive number by Optiland: ``` lens = Optic() lens.add_surface(index=0, radius=np.inf, thickness=10_000) lens.add_surface( index=1, surface_type="paraxial",...