Shaped Bokeh
Rendering out-of-focus highlights as a chosen shape by placing a cut-out mask over the lens.
Measured in
mm (aperture cut-out)
In depth
Because a defocused point of light takes the shape of the aperture, replacing the circular iris with a card cut-out makes every highlight a heart, star or any other outline. The cut-out must be small enough to become the effective aperture, which costs several stops and requires bright point sources at a good distance behind the subject.
Examples in practice
A card with a 12 mm star cut-out over a 50 mm f/1.4 turns distant fairy lights into a field of stars.