Getting really excited about a77 II

Started Mar 3, 2014 | Discussions thread
IanML Senior Member • Posts: 1,654
Re: Mirrorless pls

123Mike wrote:

If there is a tolerance, a play, rather, in the drive mechanism, it might be that the camera can not predict, when the starting of the focus motor is engaged, that when the lens actually starts changing focus. With 100% pdaf it could be that it simply observes when it starts moving, or rather, when it becomes in focus.

With CDAF, it can not really know when something is perfectly in focus, until it has seen where the peak is. It has to go past the peak point to know it has passed a peak. So with the E-mount lenses, they're super tight and have no play. The focus motor is inside, and does not have any play, and uses a precise stepping motor to drive the focus. When during a contrast focus mechanism, when that focus peak is passed, it can *quickly* backtrack.

But, with a body screw drive, there are all these gears and what not. The camera will not know "how much" to backtrack, so to say. I might have zoof back and forth performing a multi-pass, slower, ritual. Like what we see with the LA-EA1/3 adapters.

Now, why that would also be with a SAM or other in-lens motor system, I don't know. Perhaps the same deal. It can not know in advance how much to drive that motor backwards when it crosses that focus peak. Perhaps it *could* with some lenses, but not reliably across all lenses.

Does this sound reasonable?

Yes, that is certainly part of the problem. Whether it can be overcome (by suitable algorithms and very fast processing within the camera) is to be ascertained (assuming Sony is willing to devote that much resource to it). One possibility could be that the camera run a very quick dynamic test of the lens on switch-on, to be able to select the best-fit algorithm for that lens.

It may be that it cannot be overcome, but the on-sensor PDAF will prove capable of being developed to the point where it is equal to what we now have with the SLT technology.

Time will tell.

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Sony Cyber-shot DSC-HX90V Sony RX10 IV Sony SLT-A33 Sony Alpha NEX-5N Sony a77 II +1 more
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