In this article we look at the mirror lens, providing a guide to what it is for and how it works
Popular in the 1970s and ’80s, telephoto mirror lenses were cool to own and use because they were much more compact than conventional telephoto lenses. They were also competitively priced. Today, they are much less common, although Tokina and Samyang make 300mm f/6.3 designs for Micro Four Thirds and APS-C cameras.
What is a mirror lens? The basic design has been around for 200 years and was originally developed for telescopes and for projecting beams of light. Conventional refracting telephoto optics are inherently very long, but by using an arrangement of curved mirrors, an optically long telephoto lens could be fitted into a short space. In fact, the design uses both mirrors (catoptrics) and refracting optics (dioptrics), so lenses of this type are known as catadioptric, and often referred to as ‘CATs’.
But for the mirror aspect of the optics to work, you have to block the central area of the optical path to accommodate the secondary mirror. You don’t notice this in focused details, but contrasty, out-of-focus areas exhibit ring-like or ‘doughnut’ features, making bokeh distracting.
If that’s not enough to put you off, then compared to a good conventional refracting lens design, you will notice lower contrast and sharpness. And the final blow is that there is no way to incorporate an adjustable aperture, so you are usually stuck with f/8 for a 500mm lens or f/5.6 for a 250mm or 300mm lens. Nevertheless, a bit like vinyl records, there is some renewed interest in mirror lenses thanks to their affordability as used bargains, and because of their odd but distinctive characteristics.