gaffer

A lens gathers light through an opening, and a point that isn't on the focal plane arrives as a disc the shape of that opening. That's why an out-of-focus highlight behind a real camera is a circle, or a hexagon, rather than a soft grey smudge, and it's why depth of field is more than a blur. This builds the disc, so bright highlights bloom into the shape of the iris instead of dissolving.

Two things then move the focus, and both are models of a physical object rather than an envelope follower. The rattle is the one worth explaining: a camera in front of a loud PA shakes the way cameras shake, whatever the music's doing. The pressure hits an assembly with its own mass and stiffness, which answers at its own frequency and carries on answering after the hit's gone. So the rig here is four damped oscillators, one per axis, driven by the arrival of the bass rather than its level: a kick rings it, a drone doesn't move it. The lens elements are part of the same assembly, so the frame moves and the focus comes off together.

The rack is a focus puller with two marks taped on the barrel. Follow turns the Focus control into a place you're pointing at and lets the hand take its time getting there, so animating focus in the host lags the way a real pull does. Pull racks between the two marks on a cue: a bar line, a button, or the kick. Sweep never stops, hunting slowly or polling through every plane in the picture several times a second. Stutter puts a new plane on every cue, which turns focus into a rhythm part rather than a camera move.

The depth itself comes from one of five places, and they're five different things rather than five settings of one. Radial makes a field up and works on any clip at all. Luma and Alpha read one out of the picture's own channels. Split H and Split V read it out of the other half of a double-width or double-height frame, the only one of the five that can carry a proper, independent depth pass at full resolution.