Aperture Blades and Bokeh Shape Explained
Two lenses can produce identical depth of field numbers and still render background blur completely differently. One gives smooth, round highlights that melt into the background. The other gives hard-edged polygons that look mechanical and busy. The difference has nothing to do with aperture value and everything to do with a small mechanical part most photographers never think about: the aperture blades.
What Aperture Blades Actually Do
The aperture in a lens is not a single fixed hole. It is formed by a ring of thin metal or plastic blades that overlap to create an opening, and that opening can be widened or narrowed to control how much light passes through. At the widest aperture setting, the blades retract almost entirely, leaving a nearly circular opening defined by the lens elements themselves. As you stop down, the blades close in, and the shape of the opening they form becomes visible in out-of-focus points of light.
Out-of-focus highlights in a photograph, streetlights at night, sunlight through leaves, a string of fairy lights, take on the shape of the aperture opening. This is why bokeh shape is directly tied to blade design rather than to the aperture number itself.
📐 Why Blade Shape Becomes Visible
Wide open (maximum aperture): opening is defined mostly by the lens barrel, tends toward circular regardless of blade count
Stopped down partway: the blades themselves define the opening shape, and their count becomes visible in bokeh highlights
This is why a lens can produce round bokeh wide open but polygonal bokeh at f/4 or f/5.6, even though nothing else about the lens has changed.
Blade Count and Bokeh Shape
The number of blades in the aperture mechanism determines how many sides an out-of-focus highlight will show once the lens is stopped down enough for the blade shape to become visible. Fewer blades produce more obviously polygonal shapes. More blades approximate a circle more closely.
| Blade Count | Bokeh Shape When Stopped Down | Common In |
|---|---|---|
| 5 blades | Clear pentagon shape | Older budget lenses, some vintage glass |
| 6 blades | Clear hexagon shape | Many kit lenses and budget primes |
| 7 blades | Heptagon, less obviously geometric | Mid-range primes and zooms |
| 9 blades | Near-circular even stopped down moderately | Most modern professional lenses |
| 11 blades | Essentially circular at all but the smallest apertures | High-end modern primes, flagship lenses |
More blades generally means smoother, rounder bokeh across a wider range of apertures. This is one of the genuine, non-marketing reasons that premium lenses often cost more than budget alternatives with similar maximum apertures and focal lengths: the blade count and quality directly affects the character of the blur, which is a large part of what people are paying for in a fast prime lens.
Rounded vs Straight Blade Edges
Beyond blade count, the physical shape of each individual blade matters. Straight-edged blades produce a bokeh shape with clearly defined flat sides, a true polygon. Curved or rounded blade edges soften those flat sides, making the resulting shape closer to a circle even at apertures where the blade count would otherwise be visible.
Straight-Edged Blades
Even with a high blade count, straight edges produce a bokeh shape with distinct flat facets, particularly visible on bright point-source highlights like string lights or specular reflections. Common on older lens designs and some budget modern lenses.
Rounded (Curved) Blades
Curved blade edges smooth out the polygon shape considerably, producing bokeh that reads as round or nearly round even at apertures like f/4 or f/5.6 where a straight-bladed lens of the same blade count would show clear geometric edges. This is the standard in most modern premium primes.
Cat's Eye Bokeh: A Different Phenomenon
Bokeh highlights toward the edges and corners of a frame often take on an elongated, lemon or cat's eye shape rather than the round or polygonal shape seen in the centre of the image. This is not caused by the aperture blades themselves but by a separate optical effect called mechanical vignetting, or optical vignetting, where the lens barrel or internal elements physically block part of the light cone reaching the edge of the sensor at an angle.
| Cause | Effect on Bokeh | Where It Occurs |
|---|---|---|
| Aperture blade shape | Polygon vs circle | Uniform across the frame |
| Mechanical/optical vignetting | Cat's eye / lemon shape | Frame edges and corners specifically, worse wide open |
Cat's eye bokeh is most pronounced at maximum aperture and reduces as you stop down, since stopping down reduces the size of the light cone entering the lens and lessens the amount that gets clipped by the barrel toward the edges. This is a separate phenomenon from the polygon effect of blade count, and the two can occur simultaneously: round or slightly polygonal bokeh in the centre, elongating into cat's eye shapes toward the corners.
💡 Cat's Eye Bokeh Is Often a Deliberate Aesthetic Choice
Many photographers and cinematographers, particularly in video and anamorphic-style work, deliberately favour lenses with pronounced cat's eye bokeh for the swirling, dynamic look it creates in busy backgrounds. It is not a defect to be corrected but a genuine optical characteristic that some photographers select for specifically, while others prefer lenses engineered to minimise it for the cleanest, most neutral bokeh possible.
Why Some Lenses Are Marketed on Blade Count
Lens manufacturers frequently advertise blade count as a selling point, particularly for portrait and fast prime lenses where background blur quality is a primary purchase consideration. An 11-blade aperture is a meaningful, measurable specification that directly predicts how round the bokeh will remain as you stop down from maximum aperture, unlike more subjective marketing language.
| Lens Category | Typical Blade Count | Why It Matters |
|---|---|---|
| Budget kit zoom | 5–7 blades | Cost reduction, bokeh quality not a primary design goal |
| Mid-range prime (f/1.8) | 7–9 blades | Balance of cost and bokeh quality |
| Premium prime (f/1.4 and wider) | 9–11 blades | Bokeh quality is a primary selling point and design priority |
| Cinema lenses | Often 9–14 blades, sometimes with specific shape design | Bokeh character is critical to the cinematic look |
How This Interacts with Depth of Field
Blade count and shape do not affect the size or extent of depth of field, which is governed purely by aperture value, focal length, and distance, as covered throughout this site's DOF calculator and related guides. What blade design affects is purely the character and shape of the blur within the out-of-focus zones that DOF calculations define. Two lenses can share identical DOF at f/2.8 and identical framing, and still produce visibly different-looking backgrounds purely due to blade geometry.
📐 Calculate DOF, Then Consider Your Lens's Blade DesignChecking a Lens's Bokeh Character Before Buying
Blade count is usually listed in a lens's official specifications, but the practical bokeh result depends on the combination of blade count, blade curvature, and overall lens design, which specifications alone do not fully capture. Searching for sample images or bokeh test comparisons for a specific lens model, particularly images showing out-of-focus string lights or similar point sources at various apertures, gives a far more reliable picture of actual bokeh character than the blade count number alone.
Final Thoughts
Aperture blades are a small mechanical detail that materially shapes how a photograph feels, independent of the depth of field numbers that dominate most technical discussions. More blades and rounded edges generally produce smoother, more pleasing bokeh across a wider aperture range, while fewer straight blades produce more obviously geometric shapes as you stop down. Neither is objectively better, since geometric bokeh has its own aesthetic appeal in certain contexts, but understanding the mechanism explains why two lenses with the same maximum aperture can render a blurred background so differently.