Reflex
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Reflex is Nikon's term for catadiatropic or mirror lenses, also known as reflectors, especially in astronomy. A reflex lens is, essentially, a Newtonian telescope, where light is reflected generally via two mirrors, into the camera. The curvature of the main mirror sets the optical characteristics of the lens.
Mirror lenses have the following key characteristics
- small size and light weight, compared to a conventional, refracting, lens of the same focal length
- fixed aperture, generally f8 or even f11
- ring-shaped out-of-focus highlights, as opposed to the blob-shaped conventional out-of-focus highlights.
- incompatibility with normal autofocus, because of the way curved mirrors affect the phase of light
- filters usually behind the lens, rather than in front of it
In principle, a mirror lens could have a very large aperture, but this would dramatically increase its overall size and weight, to the point where there was no advantage over a fast conventional lens. However, size does not increase markedly with focal length, making a 500mm f8 mirror lens scarcely larger than a 17-55 DX zoom.
Because they are not compatible with autofocus, Nikon reflex lenses are no longer in production, although they can be had relatively cheaply second hand. However, when used with a DX camera, they are difficult to focus and the effective focal length of 750mm or more makes them hard to use.
