A mirror lens, technically called a catadioptric lens, features a unique construction that combines traditional glass lens elements with mirrors. Mirrors positioned at the front and rear of the lens fold the light path, allowing the lens to achieve a long focal length in a much shorter and lighter body compared to conventional lenses.
This design results in a lens that is typically wider in diameter but half the length of a comparable standard telephoto lens, making it easier to handle and transport. Mirror lenses are popular in wildlife, sports, and astrophotography, where long focal lengths are needed without the bulk of traditional telephoto lenses.