(Editor’s note: Tim Grey returns with another guest post. This one answers why the blades that form the aperture in a lens makes a difference.)

There are many reasons to choose one lens over another, both when making a new purchase and when choosing which lens to use for a particular photo. But have you ever stopped to consider how many blades there are in the aperture assembly of a given lens? You might be surprised at the impact the structure of the lens aperture can have on your photos.

Aperture Basics

The lens aperture is designed to regulate the flow of light through the lens. When the aperture is stopped down to a smaller opening (a higher f-number like f/16 or f/22), less light passes through the lens to the image sensor (or film) in the camera. By opening up the aperture to a larger opening (a smaller f-number f/2.0 or f/1.4), more light is able to reach the image sensor. In other words, you can use the lens aperture to adjust the overall exposure for a photograph, along with adjusting the shutter speed and ISO setting. There’s more to choosing an aperture than just setting the exposure. The selected aperture also has an impact on depth of field. Stopping down the aperture creates more depth of field in a photo, and opening up the aperture reduces depth of field.

But there’s even more to the aperture than that.

5 bladed aperture photo by Kevin Ames
An aperture with five blades.

Blade Count

In theory, the lens aperture forms a circular shape. In actual fact, that shape is not a perfect circle, in part because the lens needs to be able to use the same blades to produce apertures of different sizes. The result is that the aperture shape is more like a polygon or a hexagon or an octagon (you get the idea) than a perfect circle. The “imperfections” in the circle, where two neighboring aperture blades overlap, is what creates the rays of a sunburst effect when the lens is stopped down and a sharp light source is in the frame. And, of course, the number of those imperfections depends on the number of blades used to assemble the lens aperture. In other words, the number of blades in the lens aperture assembly determines how many rays will emanate from the light source in the photo. But the relationship isn’t actually that direct. As it turns out, if the lens has an even number of aperture blades, the starburst effect produced by that lens will include the same number of rays in the starburst as blades in the aperture. So, for example, a lens with a lens aperture consisting of eight blades will produce a starburst effect with eight rays.

An even number of aperture blades makes a starburst with the same number of rays as the number of blades.
8 blades produces 8 rays in a sunburst.

However, if the lens aperture consists of an odd number of blades, that lens will produce a starburst effect with twice as many rays as there are blades in the lens aperture. So, for example, a lens with nine blades in the lens aperture assembly will produce a starburst effect that includes eighteen rays.

An odd number of aperture blades in a lens produces a starburst that has twice the number of rays as the blade count.
An odd number of blades doubles the rays. Here the aperture has 9 blades.

Making the Choice

Of course, not all photographers are fond of the starburst effect in their photos in the first place. For them, the best approach might be to avoid scenes with sharp light sources, or to make sure not to stop the lens aperture down too much when those types of light sources can’t be avoided. With most lenses you won’t see a significant starburst effect unless you stop the lens down beyond about f/8, with a stronger effect at around f/16 to f/22. Other photographers, myself included, actually like the starburst effect, even though it may seem to be a bit of a cliché. Whenever the scene I’m photographing lends itself to a starburst effect, such as a night view of a city skyline or when the sun can be included in the frame on a clear day, I’m likely to stop down the lens aperture to achieve a starburst effect.

By keeping in mind that the number of blades in the lens aperture will have an impact on the nature of the starburst effect that lens is capable of creating, you can make a choice about which lens to use based on the desired starburst effect.

I’m not sure that this issue will be an important enough factor to impact a photographer’s purchasing decision. For myself, I’ve never actually decided which lens to purchase based on how many blades the lens aperture included.

However, on many occasions I have chosen which lens to take with me on a given trip, and which lens to use when photographing a particular scene, based on the lens aperture blade count. Sometimes I like the more “sparkly” look of a starburst effect with many rays of light, and other times I want a cleaner effect.

An Informed Decision

When it comes to choosing which lens to use for a photograph, it is helpful to consider the many ways that choice might impact the final image. In the case of creating a starburst effect in a photo, the number of blades is worth considering.