A polygonal shape sitting inside every out-of-focus streetlight in a frame has nothing to do with the glass a lens is made from. It's the shadow of a mechanism: the iris diaphragm, a ring of metal blades that open and close to set the aperture, projected directly onto the sensor for any point of light that isn't in focus.
Why the Iris Defines the Shape
A defocused point source doesn't land on the sensor as a point — it spreads into a disc, and the boundary of that disc is the shape of the aperture opening the light passed through. A 7-blade iris produces a heptagonal disc. A 9-blade iris produces a nonagon, closer to round. Blades with a curved edge, common on higher-end lens designs, round the polygon out further still even at a fixed blade count. More blades or more curvature means the geometric opening approximates a circle more closely, which is the entire reason lens makers went from 5- and 6-blade designs to 9- and 11-blade ones: rounder out-of-focus highlights were a deliberate design target, not an accident of more parts.
This is a purely mechanical effect — the blades physically clip the light bundle passing through the aperture, and the shape of that clipping is the shape you see.
Why Stopping Down Erases It
Stop the same lens down and the polygon softens, then disappears, well before the aperture closes all the way. Diffraction is why: light waves bend around the aperture blades' edges, and that bending spreads light beyond the blades' sharp geometric boundary. As the opening shrinks, the diffraction pattern grows relative to the size of the geometric image the blades are casting, until the wave behavior dominates and the polygon's corners fill in. The result is a soft, roughly circular blur that no longer reflects the blade count at all — the mechanical shape hasn't changed, but the physics that would reveal it has been overtaken by a different physics that doesn't care how many blades there are.
Why This Only Shows Up in Point Sources
None of this touches most of a defocused background. An out-of-focus leaf isn't a point source — it's an extended shape, and every point across it blurs into its neighbors' blur, smoothing the whole area into a continuous gradient with no sharp aperture-shaped edge anywhere in it. A defocused streetlight is different: a genuinely small, bright, isolated source with no neighbor to blend into, so the aperture's shape stays visible at its edge instead of averaging away. That's the actual boundary of where "bokeh shape" as blade geometry applies — bright, small, isolated highlights, not the general softness of an out-of-focus background.
Two More Shapes That Aren't the Iris
Two other bokeh artifacts get attributed to the aperture blades and aren't caused by them at all.
"Onion ring" bokeh — concentric rings visible inside a defocused highlight — comes from aspherical lens elements, which are molded rather than ground and polished like traditional spherical glass. The molding process can leave microscopic ripples on the aspherical surface, artifacts of the tooling rather than the intended curve. Under normal focus those ripples are far too small to matter. Defocused and magnified into a large blur circle, they become visible as faint concentric bands — a manufacturing signature of the molding process, not the iris.
Cat's-eye or lemon-shaped bokeh — highlights that lose their roundness and take on an oval, truncated shape toward the edges and corners of the frame — comes from mechanical vignetting. For a point off the lens's central axis, the lens barrel and the front and rear element mounts physically block part of the light cone before it reaches the aperture, an effect that gets more severe the farther off-axis the point sits. The aperture blades still cut their normal polygon, but the barrel is now cutting into that polygon too, from a different direction, leaving a shape that's neither the blade polygon nor a circle. It's why the same lens can show clean round highlights near the center of frame and increasingly clipped, oval ones toward the corners, with no change to the aperture setting at all.
For how a related out-of-focus optical effect — light bending through refraction at the aperture's own edges — connects to the softening described above, see our piece on modulation transfer function charts, which covers how a lens's in-focus performance is measured using some of the same diffraction physics.
None of these three effects — blade polygon, onion rings, cat's-eye clipping — comes from the glass doing the focusing. They come from the metal, the mold, and the barrel doing the parts of the job that focusing was never meant to cover. You can see how a lens's rendering carries into a finished grade in the Cineon studio.