Lens blur

A gaussian blur averages neighboring pixels under a smooth bell curve, so everything softens uniformly and bright points dim into their surroundings. A lens blur spreads each point into the shape of a camera aperture and preserves the dominance of bright light, so highlights bloom into defined discs or polygons while the rest defocuses around them. Gaussian is softness; lens blur is optics.

Bokeh is the aesthetic character of the out-of-focus areas of a photograph, from the Japanese boke, meaning blur or haze. It is not the amount of blur but its quality: the shape of the highlight balls, whether their edges are smooth or bright-rimmed, how gently focused areas fall off into defocus. Photographers argue about good and bad bokeh the way audiophiles argue about speakers, and the argument is entirely about these signatures.

Because the aperture is not a perfect circle. It is an iris built from overlapping blades, and when the lens is stopped down the opening becomes a polygon with one side per blade. Every out-of-focus highlight is a projection of that opening, so six blades give hexagonal bokeh, and rounded blade edges pull the shape back toward a circle. Older lenses with few straight blades produce the most obviously geometric balls.

Toward the corners of the frame, the lens barrel itself starts clipping the light path, an effect called mechanical vignetting. The clipped aperture renders as a lens or cat-eye shape rather than a full disc, squashed along the direction pointing at the frame center, so the balls appear to arc around the middle of the image. It is technically a flaw, and photographers pay for glass that has it, because it signals real optics.

Yes. The defocus is computed for every frame, so moving highlights bloom and travel as coherent bokeh shapes rather than flickering. In Modulate the Lens Blur effect runs live on images and video in the browser, and pairs naturally with grain and a vignette when the goal is a filmed look.