Diffractive Lens vs. Google Cardboard’s Asphere

As I mentioned in my last post, Google Cardboard is the interim hardware for developing Daydream apps.  The lenses look very similar to those used in the Oculus Rift development kits, although a bit faster and smaller in diameter.  The field of view is also a bit smaller.  On-axis, the MTF is nearly perfect, but off axis (with the eye still centered in the eyebox), it’s quite poor.

Now, if I make one surface diffractive and reoptimize, the MTF improves dramatically.  Much of the improvement comes from chromatic correction, including the longitudinal variety, which cannot be corrected in image processing.