David Kilpatrick wrote:These 'apertures' are often misunderstood. You have to think of an f/5.6 sensor being like the rangefinder on a Leica M, and an f/7.1 sensor being like the rangefinder on the Leica CL. It's not really to do with light, or aperture, it is like a rangefinder base difference. The M has the two windows further apart, so its rangefinder is more accurate than the CL.
Those AF 'apertures' are most deeply misunderstood by the vast majority. In fact, the PDAF has just a window of phase shifts that it's better tuned to work with. The greater the 'aperture', the better the AF performance is with the larger phase shifts that occur with the larger lens max apertures AND/OR with the longer focal lengths (because the DOF relative to focal length goes down thus increasing the phase shift).
Then if PDAF is used with the UW lenses, the issue to struggle against is the phase shifts too small (the DOF too large), and then we need very small 'aperture' AF sensors. So focusing the UW needs AF sensors capable of doing very small 'apertures' like f/22
And those 'phase shifts' I talk about here are a very simple thing - essentially the same as blur radius for a point light source. Different light paths cause a blurred spot image of a point source on a main sensor, or they cause two smaller spots shifted vs each other in the AF rangefinder.
And also quite obviously a hybrid PDAF/CDAF of sorts is a natural development to happen eventually to the PDAF systems - those 'lines/crosses' may get substituted with 'squares/rounds' containing the former, and those 'squares/rounds' may get the ability to CDAF within their areas as processing power permits.