Turn highlights into aperture discs
Bokeh is what a point of light looks like when it lands out of focus: the lens projects the shape of its own aperture onto the sensor. Every bright pixel here scatters into that shape, and overlapping scatters add together the way real light does.
Highlight threshold decides what counts as a light. Disc diameter is a percentage of the shorter image side, which keeps the preview and the full-resolution export identical in proportion.
Blade count changes the shape
Wide open, most lenses have their blades retracted into a near circle, so out-of-focus lights are round. Stopped down, the blade edges close in and the same lights become polygons.
Set Aperture blades to 0 for the round look, 6 for the hexagons of a common consumer lens, or 9 for the softer polygon of a lens with more blades. Every polygon shares one orientation because all highlights pass through the same aperture.
Only surplus light scatters
The discs are drawn from the light a highlight carries above what the background defocus already spreads. A lamp against a dark window is almost all surplus and produces a full disc. A pale wall spreads into itself, leaves nothing over, and stays a wall.
That is why raising Disc brightness on an evenly lit photograph changes very little. The source needs concentrated light, not bright light.
Background defocus
The slider blurs the whole image before the discs are added, on squared channel values so bright detail glows instead of averaging into grey. It creates the background-lights treatment directly, but it also softens the subject because there is no depth information to protect it.
Set it to 0 when the source is already shallow, and raise it when the source was shot at a small aperture and everything is uniformly sharp. Around 18 softens fine background detail while leaving major edges recognizable.
Example: string lights behind a subject
Load a photo of a person with fairy lights behind them. Set Highlight threshold near 205 so only the bulbs qualify, Disc diameter around 6, and blades at 0.
Raise Disc brightness until the discs read as light rather than as grey shapes. If discs appear over the subject’s face, the threshold is catching skin highlights, so raise it before touching anything else.
Limits
Nothing here recovers depth. The tool cannot know that the lights are behind the subject, so a bright highlight on a foreground object gets a disc exactly like a background bulb does.
Very large discs over a dense field of lights still merge into a wash. Energy is rolled off as it accumulates, so the result saturates toward white instead of clipping into hard edges, but the individual shapes are gone. Reduce Disc diameter before reducing brightness.