Optics June 29, 2026 4 min read

Aperture Blades and Bokeh Shape Explained

Two lenses can have identical depth of field and still draw a blurred background differently. One gives smooth round highlights that melt away, the other hard-edged polygons that look busy. The cause is a small mechanical part most photographers never think about: the aperture blades.

Why the shape shows only when you stop down

The aperture is not a fixed hole. It is a ring of overlapping blades that close to form an opening. Wide open, the blades are fully retracted and the opening is set by the lens barrel and elements, which is round. As you stop down, the blades close in and become the limit, so their shape appears in out-of-focus points of light such as streetlights, sun through leaves or fairy lights. That is why a lens can give round highlights wide open and polygonal ones at f/4 or f/5.6, with nothing else changed.

How round is a polygon?

With straight blades, the opening is a regular polygon with as many sides as blades. How far it departs from a circle is easy to work out: the distance across the flats compared with corner to corner is the cosine of 180 degrees divided by the number of blades.

Blades	Flats compared with corners	Typical lens
5	19 percent narrower	Older budget lenses, some vintage glass
6	13 percent narrower	Many kit lenses and budget primes
7	10 percent narrower	Mid-range primes and zooms
8	8 percent narrower	Mid-range and some premium lenses
9	6 percent narrower	Many modern professional lenses
11	4 percent narrower	High-end primes

So a 6-blade lens with straight edges is a clear hexagon, 13 percent narrower across the flats, while a straight-bladed 11-blade lens is only 4 percent off round, which most people read as a circle. Curved blade edges round the shape further, so a 7-blade lens with curved blades can look rounder than a 9-blade one with straight blades. Blade count is a good guide, not the whole story.

Sunstars

The same shape also decides the spikes on bright lights when you stop down to f/11 or f/16. A lens with an even number of blades gives that many spikes, and a lens with an odd number gives twice as many. So 6 blades give 6 spikes, 8 give 8, 7 give 14 and 9 give 18. It is a free compositional tool on a tripod at night, and the night photography post uses it.

Cat's eye bokeh

Highlights near the corners often stretch into lemon or cat's eye shapes. That is not the blades. It is optical or mechanical vignetting, where the lens barrel and internal elements cut off part of the light cone reaching the edge of the frame at an angle. It is strongest wide open and fades as you stop down, because a narrower cone is clipped less. The two effects can combine, with round highlights at the centre and cat's eyes at the edges. Some photographers and filmmakers like the swirling look, and others prefer lenses that minimise it.

Shape is not the same as quality

Blades set the outline of a highlight, not what is inside it. Some lenses draw discs with bright edges, others with soft ones, and some show an "onion ring" pattern from aspherical elements. That depends on the optical design and aberrations. A few lenses use an apodization filter to give very smooth edges, at the cost of light, such as the Sony FE 100 mm f/2.8 STF and Fujifilm's XF 56 mm f/1.2 APD. For most lenses, the best check is a test shot or sample image of out-of-focus string lights at several apertures, since the spec sheet will not show it.

What it does not change

Blades do not affect the size of the zone in focus, which depends on the aperture, focal length, distance and sensor, as the depth of field basics page explains. They do not change how much blur there is either, which is the subject of the bokeh guide. They change what the blur looks like.

Marketing often quotes blade count, and it is a measurable spec, but a high number does not guarantee smooth blur. Look at sample images, and check them at the apertures you actually use.