[Firerouge]

I wonder this as well, perhaps it's a problem related to manufacturing shrinking sized concentric steps?

If my understanding is correct, fresnel lenses effectively operate as a larger lense surface by concentrating light with flat or curved steps getting smaller in size packed towards the core.

[hinkley]

Unless I’m mistaken, you should be able to invert a fresnel lens stamp, make the same plastic lens and then silver it, so now it’s a concave mirror instead of a convex lens.