How does on sensor PDAF work?

Started May 28, 2013 | Discussions thread
EinsteinsGhost
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Re: How does on sensor PDAF work?
In reply to Dave Lively, May 28, 2013

Dave Lively wrote:

Has anyone seen an explanation of how this works?  I would really like a camera that combines the speed and tracking ability of PDAF with the absolute accuracy of CDAF.  But just because OSPDAF and PDAF have "Phase Detect" in their acronym does not mean they are the same thing.  I can find a lot of websites that explain how conventional PDAF in a SLR works.  But that system absolutely requires a mirror positioned before the focal plane.  So OSPDAF must work in some very different way.

Phase detect AF, first implemented in production SLR in 1985 by Minolta, uses the same premise as on-sensor PDAF: Phase detect. The difference is in how to get "phase detect".

In conventional mirror-based SLR system, the mirror reflects a part of the light to PDAF sensors either on top or bottom. If you can take these PDAF sensors out from the conventional positions and place them on the sensor, you have the same functionality without the mirror.

I see a lot of speculation about how OSPDAF is disruptive technology that will allow mirrorless cameras to have the same or better AF performance as SLRs in the near future.  But from what I have seen so far it appears to be helpful but not a game changer.  Without the mirror PDAF does not seem to work nearly as well.

Because the technology is in its infancy stage at this time. There are technical challenges to be addressed, but it may be only a matter of time before a mirror-less PDAF system matches a 28-year old mirror based-PDAF convention.

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