apersson850 wrote in post #15695580
The difference isn't about accuracy, but about sensitivity. Linear focus point are less likely to detect a useful contrast. When they can't, the aren't accurate either, but when they can, they are just as accurate. Their single line is the same as one half of a cross type point.
I guess it makes sense to call it sensitivity
No it doesn't. The thing that makes phase detection AF so quick is that although the subject is out of focus, the camera can measure both in which direction and how far off immediately. So it knows where to drive the lens and just does that. The description above fits contrast AF, but that's not what you use when you use the AF points you see in the viewfinder.
Yes, it does; how would it know the direction? Here's a bit taken from Cambridge in Colour:
"The process of autofocusing generally works as follows:
(1) An autofocus processor (AFP) makes a small change in the focusing distance.
(2) AFP reads the AF sensor to assess whether and by how much focus has improved.
(3) Using the information from (2), the AFP sets the lens to a new focusing distance.
(4) The AFP may iteratively repeat steps 2-3 until satisfactory focus has been achieved."
The AF system of a modern camera can repeat the above process hundreds of times per second, so maybe it seems like it's focusing in a single motion, but it's not, it's just happening too fast to notice. The only AF systems that can truly see the direction of focus, are on 5-6 figure lenses used for HDTV broadcast, using two AF sensors spaced physically apart.
No it doesn't. When using multiple point in One Shot AF mode, the camera will focus on the closest subject with proper contrast, not average focus out among all the points that find any contrast. So if eight points see something in the background, and sees it well, the camera will still focus on a point in the foreground, if there's one single point finding a subject there.
I was referring to cameras with A-DEP, I should probably clarify that in my post. https://photography-on-the.net/forum/showthread.php?t=270557
Edit: Might as well drop the link to CiC as well: http://www.cambridgeincolour.com …ials/camera-autofocus.htm