i also wanted to note some things about how complicated this whole subject is.
for example, it's been pointed out that subject detection and af are two different things, which i think is true, but obviously they are intended to work closely together.
as i understand it, we have af sensors built into the image sensor so from a basic perspective, we could assume that subject detection will basically tell the camera which af sensor to use.
but....
as i understand it, it also uses contrast detection.
so basically the camera is using either the af sensor, or contrast detection, or some combination of the two. does it vary how much (like "in this situation i'll use 60% af sensor and 30% contrast detection")?
so one big thing that comes to mind is apeture impacts both subject detection and, we would assume, contrast detection. we can see this when cameras struggle where everything is so out of focus the camera can't tell which way is the right way to get it into focus, and thus has to hunt back and forth.
another situation to think about. so if you have subject partially obscured by foreground foliage, if the way it's working is subject detection decides which af sensor to use and it relies primarily or mostly on the af sensor, there's a fair chance that the af sensor is going to pick up the branch, not the subject since it's going to just find whatever it first sees.
so the only way this works is that it has to switch to favoring contrast detection?
the other big thing i wanted to point out, and this is on my mind a lot based on my subject matter is that we can't forget a big part of af is PREDICTION. when something is coming directly at you, afaik, the camera is basically keeping track of how the subject is moving and moving the focus predictively to meet the subject where it thinks it will be. this seems to work well for items that have a relatively constaint velocity. however, i think this means that we should expect focus misses when things abruptly change velocity (like jumping dogs
).
this has led me to try to keep af engaged as much as possible to let it get the best idea of how the subject is moving. i also suspect the longer you keep af engaged, it seems to get a better idea about what your subject is
but basically, the more you understand about the af and subject detection systems, the more you realize how complicated it is and how much we don't know
all i could do is try to understand as much as i could about how things worked (thanks Steve for the AF books and videos), then try to guess what might have gone wrong and try to change things up and try again.
