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Mastering the Mysteries of Autofocus

Designing a fast, efficient autofocus system was one of the challenges facing the designers of Sony’s super-compact RX100 series of cameras, including the RX100 IV. The mirrors found in traditional SLRs (and Sony’s own A-mount SLT cameras) provide a distinct advantage: they make it possible to direct some of the incoming illumination to a separate electronic autofocus system that’s fast and accurate. At the moment of exposure, focus is locked, and the mirror flipped out of the way (except in fixed mirror SLT models), allowing the light to pass through the camera body to the film or digital sensor.

Fixed-lens cameras like the RX100 IV have no mirror, so there’s no way to insert an outboard AF system into ...

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