General formula to design a freeform singlet free of spherical aberration and astigmatism : Comment.

In their article -Appl. Opt. 58, 1010_1015 (2019)- Gonz\'alez-Acu\~na et al claimed:"an analytical closed-form formula for the design of freeform lenses free of spherical aberration and astigmatism." However, as we show here, their formula can only be applied when the object and image are both real and the image is inversed, additionally, the refractive index in the object and image media is the same. Here, we present the complete solution of this particular formula.