Bokeh

Out of focus points of light stop being points and become shapes. The background turns into a field of soft discs, which is most of what people mean when they call footage cinematic.

The word comes from the Japanese boke, meaning blur. It describes the quality of the out of focus area rather than the amount of it, so two lenses at the same aperture can render the same background very differently.

The shape of a highlight is the shape of the aperture. A round iris gives round discs. A nine bladed iris gives faint nine sided shapes once the lens is stopped down at all.

Lenses with unusual optics give unusual bokeh. An anamorphic squeezes the discs into ovals. Mirror lenses render rings with hollow centres. Old Petzval designs swirl the highlights toward the corners.

Cat eye bokeh appears near the frame edges, where the lens barrel clips the light path and cuts one side off each disc. It is strongest wide open and closes up as you stop down.

Use it when the background has lights in it worth turning into shapes: a street at night, a window, a string of bulbs, water catching the sun.

Leave it alone on a plain background. Bokeh with nothing to render is just a soft wall, and asking for it wastes the run.

Shallow focus is the decision that makes bokeh possible. It thins the plane of sharpness. Bokeh is what the blur that results actually looks like.

The test is what is behind the subject. Shallow focus against a flat wall shows almost no bokeh. Put point lights back there and the same lens at the same aperture fills the frame with discs.

The background comes back soft and empty, because there was nothing in it to become bokeh. Put point lights behind the subject and say so in the prompt.

The second failure is discs that look pasted on, all the same size and evenly spaced. Real bokeh varies with distance, so name how far back the lights are and let them sit at different depths.