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David Busch’s Sony Alpha a7R II/a7 II Guide to Digital Photography by David D. Busch

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8

Mastering Autofocus Options

The mirrors found in traditional SLRs had two distinct advantages: they allowed previewing an image through an optical viewfinder, and made it possible to direct some of the incoming illumination to a separate electronic autofocus system. At the moment of exposure, focus was locked, and the mirror flipped up out of the way, allowing the light to pass through the camera body to the film or digital sensor. Autofocus was fast, and reasonably accurate.

But the mirror system had its drawbacks, which were generally eliminated when mirrorless systems like the current Sony a7 II series were introduced. Moving mirrors are noisy and bulky, and increase the distance between the lens mounting flange and the film or sensor, ...

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