a57 Face Detect fail

Don't know which 'inferior' cams you have in mind, but cameras with on sensor CDAF can use the whole sensor for AF, so a face hardly has the chance to escape the AF sensor area - the image sensor is the AF sensor - generally speaking.

In a live view SLT, the PDAF sensors only cover parts of the frame, so a detected face can 'escape' the AF area if it doesn't fill it sufficiently. Small faces in the corners or 'between' sensors may be out of tracking, as there are no PDAF sensors available.

I hope this helps!

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Cheers,
Ralf