By Joe Lewitt
A compact, relatively inexpensive and lightweight option for super-telephotography is the Reflex or Mirror lens. This is based on
the same catadioptric optical design as astronomical telescopes, with the light path passing through lenses and reflecting back and forth on mirrors within the lens body, thereby multiplying the focal length in a compact package. As part of this design, there is no adjustable aperture.
The great advantages are the compactness of the lens and the fact it does not have multiple glass lens elements and therefore no chromatic aberration. Images are crisp and distortion free.
There are, however, some down-sides. Firstly a fixed aperture that is quite small, typically f8, meaning it can be used to best effect in bright sunny conditions. The exposure adjustment is made only via shutter speed, or in some cases the insertion of a rear mounted Neutral Density filter if conditions are too bright.
Secondly, due to the small front mirror placed in the middle of the front lens element, the
Bokeh highlights are represented as ‘dough-nut’ shapes, and sometimes out of focus objects can appear a bit wiry. This is not an issue when photographing an object against a clear background such as the sky, and it can of course be used as a deliberate artistic effect when photographing something silhouetted against a sunlit body of water for example.
Typically Reflex lenses will be 500mm focal length, meaning that on APSC digital sensors the cropping effect gives you the equivalent of a whopping 750mm. At that sort of focal length a monopod or tripod is useful as the tiniest camera shake is really magnified. There are larger mirror lenses going as high as 1,000mm.
My own experience is with the Minolta 500mm/f8 Reflex now re-badged but identical in the current Sony version. It is the only autofocus Reflex ever made, and couples with the Sony Alpha DSLR range of cameras that all have built-in anti-shake in the camera body, making it a great combination. The attached photos are all taken with this lens.