What is a mirror lens?
According to my friend Leonard Foo of MIR...
"You can called a Reflex Lens a poor man's ED lens.
In a reflex lens, light path is 'folded' by the mirror surfaces.
This permits the design of lens type permitting a significant reduction in size and weight as compared to conventional lenses of equivalent focal length.
Since the light path is folded, another benefit is the virtual elimination of chromatic aberration, this provides images taken with mirror lens with superior sharpness and contrast!"