You’ve probably seen it in the lens specifications: “number of diaphragm (or aperture) blades” and “circular aperture”. What does this refer to, and how does it affect your photos? Read on to find out!

 

The basics: What are aperture blades and what do they do?

Have you ever taken a close look into the “eye” of your lens? If you have, on most lenses, you’ll probably see something like this when your camera is off.:

Aperture blades, also known as diaphragm blades, are the blades that form the opening in the middle of your lens. They control the size of the aperture: the hole in the lens that allows light to reach the camera.

When you increase your f-number (“narrow the aperture”), the aperture blades extend to narrow the opening so that less light enters the camera.
When you reduce your f-number (“open up the aperture”), the aperture blades retract, increasing the size of the opening so that more light enters the camera.

Fun fact: Full-aperture metering

Why was the above image taken when the camera was off? That’s because when the camera is on, the aperture blades are likely to be fully retracted (wide open). Depending on your lens, you might not even be able to see them!

This happens because Canon cameras perform full-aperture metering: regardless of your aperture setting, the aperture stays wide open until you release the shutter. This ensures that the maximum amount of light reaches the image sensor. More light means more information, which helps with functions like autoexposure and autofocusing!

Also see:
Can A Fast Lens Really Make It Easier To See Through The Viewfinder?

 

At maximum aperture, the number of aperture blades doesn’t influence bokeh…

Here’s a close-up of two lenses with the aperture blades at maximum aperture. In both, the aperture blades are fully retracted.

What do you notice about the aperture opening?

Not related to the size difference
You’ve probably noticed the size difference. This is because the lenses were designed with different maximum apertures (f/4 and f/1.2 respectively), and this has nothing to do with the number of aperture blades. If you have a 50mm f/1.8 lens, you’ll see something like the second example. The number of aperture blades varies even among lenses with the same maximum aperture!

Same circular shape
Also notice the shape of the opening: Regardless of whether the lens has 9 or 10 blades, at maximum aperture, the aperture blades on both form a circular shape.

 

Test shoot: Bokeh lights

The following images were shot on three different 50mm prime lenses with different numbers of aperture blades:
- RF50mm f/1.2L USM (10 aperture blades)
- RF50mm f/1.8 STM (7 aperture blades)
- EF50mm f/1.8 II (5 aperture blades)

All the images were shot:
- At the lens’ maximum aperture
- From the same shooting position
- In manual focus mode, with the focus set to the lens’ closest focusing distance.

Regardless of the number of aperture blades, the shapes of the bokeh lights are the same.

 

Know this: Why are the sizes of the bokeh lights different?
The sizes of the bokeh lights are different, but that is expected due to the following factors that affect the depth of field:

Maximum aperture difference: The f/1.2 image predictably has bigger bokeh lights due to the shallower focus.  (See: Lens Basics #3: Creating Bokeh)
Difference in closest focusing distance: The EF50mm f/1.8 II has a longer closest focusing distance (45cm) compared to the RF50mm f/1.8 STM (30cm). This means that the lights in the background are more out of focus on the RF lens and therefore look bigger.

 

...but it does at narrower apertures

When we use a narrow aperture, the shape formed by the aperture blades becomes less circular.

The above image was deliberately placed out of focus and shot at f/16 on a lens with 7 aperture blades. Notice how the bokeh lights form a 7-sided polygon, reflecting the shape of the aperture opening.

Impact on starbursts

There’s another thing that happens at a very narrow aperture (f/11 or f/16 onwards), especially when you shoot with point light sources in the frame.

Yes, starbursts! You might have noticed them if you’ve ever tried taking long exposures of scenes with street lights at night, such as vehicle light trails.

Take a look at the following examples, all shot at f/16 and 50mm on lenses with different numbers of aperture blades. What do you notice about the number of starburst points?

Test shoot: Starbursts

5 aperture blades

7 aperture blades

9 aperture blades

10 aperture blades

Number of aperture blades	Number of starburst points
5	10
7	14
9	18
10	10

Why is this so?

This has to do with how starbursts are created.

At narrow apertures (high f-numbers), the aperture opening is small and the aperture blades create corners. Light passing through the small opening bends (diffracts) across these corners, producing the starburst effect.

As a general rule, if there is an odd number of aperture blades, the number of star points is twice the number of blades.

If there is an even number of aperture blades, the number of star points equals the number of blades.

Starburst tips: Getting the cleanest starburst

1. Remove your UV filter
You need a strong, concentrated light source to create the starburst. UV filters might diffuse the light entering the lens, preventing points from appearing clearly.

2. Make sure your lens is clean and dry, including the rear element
Moisture, dirt, or smudges on the lens will also bend or diffuse light, affecting the clarity of the points. Don’t forget to clean the glass at the back of the lens too, and make sure you don’t accidentally leave fingerprints!

3. Look for point light sources
The shape of the light source affects the shape of the starburst centre.

In this image, three different types of light sources produced starbursts: the street lamps, the tiny red lights on the bridge, and if you look really closely, the vintage-looking second light from the left. Which source do you think gives the nicest starburst? What makes it different?

Failed shot

Smudges on the lens have blurred the starburst points of the street lights in front. If you want nice starbursts, it’s always good to have a lens cloth, lens cleaning liquid and blower on hand. Also notice how the shape of the lights have affected the shape of the starburst.

The shape of stopped-down bokeh

The more the aperture blades, the more circular the opening appears even at small apertures. This is useful for scenes where you want round bokeh lights (also known as bokeh circles or bokeh balls) but can’t use an aperture that’s too wide.

7 blades, f/8

10 blades, f/8

The f/8 bokeh created by 5 aperture blades is distinctly polygonal. In comparison, the one created by 10 aperture blades resembles a circle. We only notice the edges when we look closely.

 

Summing up: Bokeh and starburst fans, take note

The number of aperture blades starts to matter as you stop down the aperture:
- Many aperture blades: more circular stopped-down bokeh
- Fewer aperture blades: visibly polygonal stopped-down bokeh
- Even number of aperture blades: twice the number of starburst points
- Odd number of aperture blades: same number of starburst points

L-series lenses usually have 9 or 10 blades. Non-L-series lenses mostly have 7 blades
To ensure rounder bokeh even at narrower apertures, many professional grade (L-series) RF and EF lenses have 9 or even 10 aperture blades. Meanwhile, most non-L lenses have 7 aperture blades, although some EF lenses may have 6 and others 8.

If bokeh is a big priority to you, and you want bokeh balls to appear as round and smooth even when stopped down, a lens with more aperture blades will be better.

If you’re a starburst fan, it really depends on which starburst effect you like better! If you like starbursts with fewer points, it may be worth keeping your older non-L-series lenses even when you upgrade. 
Alternatively, you can get star effect filters to use, but be aware that they can affect the overall image sharpness!

Learn more about lenses and techniques to make the most of them in:
In Focus: Lenses FAQs