arduluth
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jankap wrote:
But what is the reason for this? In the vertical direction there are sensor points too, I suppose.
Is the reason, that the vertical dimension of the sensor is less (24mm)?
Jan
Perhaps the programmer of the camera software thinks in one direction only?
It's a matter of the algorithms being used to get fast AF. CMOS sensors lack a global shutter - data is read row-by-row. I can't remember all the numbers now, but readout time on a lot of the Sony FF, APS-C, and MFT sensors is ~1/13 sec. The sensor in the A7S/A7SII has a readout speed of ~1/30, which is faster than most.
If it takes 1/13s to fully read from the sensor, why doesn't AF always take at least 1/13s? Because the CDAF system is using that data, row-by-row as it's read. I'd wager there's some row skipping on the first pass. The CDAF system uses the information in adjacent rows, but it can't see into the future. Speed is achieved by making a best guess as to when you've hit focus on that point. There's a balance between focus errors caused by these optimizations and reliable accuracy.
Imagine doing a cross-word puzzle as fast as you can while being limiting yourself to scanning rows for words, left to right. You'll be able to recognize horizontal words more quickly and reliably than vertical words. You're still able to find vertical words, but it'll require more time and be a bit more error prone - the latter because your algorithm might make assumptions about the beginnings of words to speed things up.
This is a best guess informed by working with and programming embedded systems, as well as observing camera behavior.
EDIT: @darrellc managed to explain much more succinctly. Story of my life. :P
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