Why do pro cameras still need to pull focus?

Question

Big productions using pro cinema cameras (RED Weapon etc...) still use manual focus and still have someone who's sole job is just to pull focus for a camera.With today's technology, I would always trust auto-focus over my own eyesight. I know in some instances focal length is literally measured from the sensor which is probably more accurate - but is that extra effort actually noticeable? I also know you could argue cinematic effects such as "racking focus" need manual, but you could still have Auto/Manual setting like DSLRs do.So why do we still have focus pullers and manual focus in productions with top-end cameras?

Answer 1

I don't have a definite answer but I assume it's because auto-focus isn't 100% reliable.

Let me explain:

Typically, the autofocus function of a camera needs to figure out what's in focus. Easy for a human eye but for a machine it has to determine what's in-focus by determining what constitutes out-of-focus.

I haven't used autofocus on any shoots but when I'm playing around with a DSLR and I turn on autofocus it might be fast but it's not on point within a millisecond. It typically has to rack a hair too far and a hair too short before getting it just right. My DSLR is older but I'm not sure we've made leaps and bounds since then.

Let's say there's a camera/lens/add-on out there with a perfect autofocus setting. Then factor in Murphy's Law on a really expensive camera/lens/autofocus-dedicated add-on and the company has now invested a couple hundred+ on a malfunctioning piece of equipment. On a larger production you're looking at spending more on the rental than your first assistant camera operator (1st a/c). On top of that, the 1st a/c has to be there anyway to swap lenses, clean lenses, etc.

Bottom line: It's still more reliable (possibly cheaper) to have a dedicated focus-puller than a machine that may or may not work 100% of the time.