Its compact size and relative low cost is possible through the use of a mirror lens system whereby mirrors are used to fold the light path.
Because of this design it is impossible to build a diaphragm into the lens and aperture is consequently fixed at f/8. Exposure is controlled through adjusting shutter speed.
Due to its light weight and compact size it is possible to hand hold the lens at shutter speeds of 1/500th sec or higher.
At the time of writing it is the only auto focus mirror lens on the market, but with the 7xi, 9xi and 600 & 700si cameras only the single centre AF sensor is utilised.
A trade off with the lens is that out of focus highlights are rendered as donut shaped highlights rather than plain out of focus blobs.
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