garyvot Offline Upload & Sell: On
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Holger wrote:
DSLRs calculate focus not only 20times a second. I think you misunderstood something here. The Sony is faster than the A7rii in doing so. But the dedicated AF system and RGB sensors calculate it much faster than the Sony (after e.g. Thom Hogan).
Interesting. This contradicts the discussion of this in DPReview:
"Interestingly, there's no setting to tell the camera how erratic or constant the acceleration (or deceleration) of your subject is - a parameter most DSLRs allow you to specify. Sony tells us this is because their system needn't rely as much on prediction, since it can simply react to the subject thanks to its 60 fps AF measurements. Our initial impressions line up with these claims: every now and then the camera might lose the subject, but it very quickly returns, something we can't say about the Canon 1D X II when its predictive algorithm fails."
While I am sure it is the case that the AF system in a DSLR is sampling data faster than 60 times per second when the subject is visible, the difference between these systems is the loss of acquisition in a DSLR when the mirror is moving through its up/down cycle 10, 12, or 14 times per second. The AF system is effectively "blind" then. Canon and Nikon have made tremendous strides over the decades in reducing mirror blackout times and improving the predictive capabilities of their AF systems, but nothing beats a near-continuous eye on subject.
Additionally it is not correct that OSPDAF is accurate per se (which is why an additional CDAF step is added, therefore Sony's designation as hybrid AF). There is always a "dead zone" when focus is achieved, as the light rays from different parts of the lens are located within the COC and no phase information is available. You need predictive focus movement algorithms (problematic with erratically moving subjects) or let the subject get minimally OOF until phase information is regained.
Yes, I am sure this is right. I spoke imprecisely. I wasn't referring exclusively to PDAF on-sensor, but the benefits sensor-based focusing generally. Hybrid systems have the benefit of being more accurate if given time and light to operate, and do not suffer from calibration issues. I am pretty sure though that the Sony isn't using CDAF during continuous tracking, and so you are right that there is room for error in the system.
The reason I am intrigued is that having suffered through inaccurate / inconsistent DSLR focusing issues for years, on-sensor focusing is a significant potential benefit to me. (I would say the same about shooting in Live View on newer Canons--would that there were articulated LCDs and/or EVFs available for the high end models!)
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