Bokeh Quality: Why “Cat’s Eye Bokeh” and “Onion Rings” Appear

Bokeh isn’t just about how much a lens blurs the background. Its appearance is determined by an entire range of optical properties—from the shape of the aperture blades to internal lens design and optical aberrations. Let’s take a look at how bokeh is rendered, what causes “cat’s eyes” and “onion rings,” and how your camera body itself can influence the appearance of bokeh.

Blurred backgrounds are immensely popular. Smartphones have offered digital background blur for years. However, professional lenses still come out ahead. You don’t have to rely on the processor to interpret the scene and create the right effect. The physical lens design plays such a significant role, resulting in various styles of bokeh.

What causes bokeh? 

The word “bokeh” comes from the Japanese word boke, meaning blur or haze. It refers to how the background is blurred. Several factors determine the amount of background blur. Greater background blur occurs when you use:

• Wider apertures (lower f-numbers)
• Longer focal lengths
• Closer focusing distances

Background blur with a Sigma 35/1.2 II lens at f/1.2 (left) and f/2.8 (right). Both shots were taken from a tripod at the exact same focus point.

Blur occurs because every point in the image expands into a larger circle (often called “bokeh balls”). These circles overlap, creating a smooth, blurry effect. 

The issue is that they aren’t always perfect circles. They can also vary in size, and their interiors can fill with unwanted textures. As a result, two lenses with identical baseball specs (like a 35mm f/2) can render the backgrounds in completely different ways and produce different bokeh.  

These optical traits are most noticeable in out-of-focus highlights, so it makes sense to focus on them. This is why when you see a bokeh review, it often features images of bright spots against a dark background. That’s where you can easily analyze how a lens renders out-of-focus light.

Picture-perfect bokeh starts with a circle

With standard lenses (as opposed to other lenses like anamorphic lenses), you get a nice, clean circle when two conditions are met: the out-of-focus light is in the center of the frame, and the ideal f-number is used. 

Cat’s eyes

Towards the edge of the frame, those circles become distorted. This effect is most pronounced on ultra-fast lenses, even in a single photo. The bokeh in the center remains perfectly round, but it flattens out toward the edges, a phenomenon known as “cat’s eye bokeh.” 

You can eliminate cat’s eyes by stopping down. As the aperture closes, the discs become increasingly regular. However, the trade-off is that they also shrink. 

Aperture blades turn bokeh into polygons

Notice a subtle detail in the last photo. Like I mentioned, when shooting wide open. That is, at the lens’ brightest setting. The bokeh balls have a beautifully rounded shape. This is due to light passing through the entire circular barrel. But when you stop down, the aperture blades inside the lens move, restricting the passage of light. The bokeh balls then take on the geometric shape of these blades.

The image above (shot with the Samyang 85/1.4 II) clearly shows the outline of its nine aperture blades, which creates nonagons (nine-sided polygons). Older or budget lenses may have as few as five aperture blades.

Higher-end lenses feature more aperture blades, sometimes up to 11. In addition, each blade may be curved, further improving the shape of the bokeh.

The design of the aperture mechanism also affects how a lens creates starburst patterns when photographing point light sources. That’s why using the highest possible number of blades isn’t always the best solution. 

When bokeh isn’t smooth: Onion rings and busy backgrounds

So far, we’ve only looked at the outer shape of these bokeh balls, but the internal texture also matters. Depending on the optical construction inside the lens barrel, concentric ring patterns, known as onion rings, can form inside.

This pronounced texture is also visible in the out-of-focus background. These lenses produce what’s known as busy bokeh.

The most noticeable difference between my lenses was small, but still visible. Left, Samyang 85/1.4 II and right, Sigma 28-200/2.8-5.6, both at f/8. The second lens adds details to the bokeh, which is why the background isn’t as blurred as it could be. 

Because of onion rings, even a lens with impressive baseline specs might produce a harsher, more distracting background than theoretically “inferior” lenses. 

Chromatic aberration

Chromatic and other lens aberrations can also affect the appearance of bokeh. Chromatic aberration causes the edges of the bokeh to take on different colors depending on their distance from you or their position in the frame. As a result, you may get bokeh circles with green or purple outlines, which isn’t aesthetically pleasing. This can sometimes be corrected in post-production.

Watch out for electronic front-curtain shutters and DSLRs

I’ve already mentioned this in another article, but it bears repeating here. A lens’ perfect bokeh can easily be ruined by incorrect camera settings. 

The problem occurs when the shutter is set to electronic first/front curtain. This has its advantages, but when shooting outdoors with very fast shutter speeds of 1/1000s and faster, bokeh circles appear half-darkened. 

Left: fully mechanical shutter. Right: electronic first curtain.

A quick fix is to switch to a fully mechanical shutter or a fully electronic shutter. Read about the advantages and disadvantages of each method in our article about shutters.

DSLR users face an even trickier situation. With extremely fast lenses, the light beam is sometimes cut off by the mirror chamber—which is essentially a tunnel in front of the sensor. It depends on the specific camera brand (each has a differently sized tunnel) and the specific lens design. Here’s an example from the Canon 5D Mark IV and Canon EF 85/1.4 IS lens:

This is not an issue with mirrorless cameras. 

Specialized lenses

There are categories of lenses that produce unique bokeh, whether intentionally or not. 

The first are telephotos lenses with two mirrors inside (catadioptric lenses).

They are common among astronomers because they significantly reduce the size of the – optical system. As a result, they occasionally appear in equipment for amateur photographers as well, though these photographers then have to put up with the unique donut-shaped bokeh.

On the other hand, certain rare lenses labeled “Defocus Smoothing,” “Apodization filter,” or similar are specifically designed to make out-of-focus highlights as smooth as possible. While the bokeh still spreads into circles, the outer edges gradually fade away. 

A comparison of the standard Canon RF 85/1.2 lens (left) and its specialized DS (Defocus Smoothing) version (right)

Lens speed isn’t everything

I could go on and on about background blur, but hopefully even this basic overview has shown you that lenses can differ significantly from one another. Unfortunately, bokeh is a characteristic that is only hinted at in the specs (aperture, number of aperture blades), but its true quality becomes apparent in actual photos. With basic lenses, you don’t need to worry about it too much, but with fast lenses, it can sometimes be the deciding factor when choosing one lens over another.