Question and Answer - How does autofocus work?
For those of you who might like to delve into a real technical article - here it is.
Here's an article explaining how split-prism focus and autofocus works.
Now I know why you need at least a certain aperture for it to work.
It's not the brightness, it's geometry. Interesting.
The secret to good photography? - Be at the right place - at the right time - with the right gear - and possess the skill to take advantage of the situation.
There is a fine line between a hobby and mental illness.