John Denker supposes that Fresnel lenses are figured to within a
wavelength of light or fraction thereof. That is not the case.
M. Fresnel invented these devices to collimate lighthouse beacons,
not to make high resolution optical images, and they don't. The
rings of the lens are cut the way they are, closer together at the
edges, in order to make them more or less the same average
thickness overall. For a plastic lens this means it can be formed
from sheet stock. (This makes the poorest quality lenses.) If you
check out an automobile taillight lens you will probably find that
part of it is a Fresnel lens. Sealed beam headlights have another
variant for collimating light, a multi-prismatic lens.