Today’s Question: How does one know how many aperture blades a lens has?
Tim’s Quick Answer: While it can sometimes be a little challenging to find this specification, I’ve found that in general the B&H Photo website is a reliable source for this information. The number of blades is listed on the Specs tab for the product page for lenses, listed as “Diaphragm Blades”. You can see an example by scrolling down and selecting the Specs tab on this page:
More Detail: Today’s question was a follow-up to information I shared about creating a starburst effect with the sun or other non-diffused light source. As many photographers are aware, you can achieve a starburst effect by stopping the lens down to a relatively small aperture size, such as f/16 or f/22.
What I find many photographers are not aware of, however, is how the number of lens aperture blades affects the starburst effect. If there are an even number of lens aperture blades, the starburst effect will have the same number of light rays as there are lens apertures.
If the lens aperture has an odd number of blades, the starburst effect will have twice the number of light rays as there are aperture blades. I’ve cited one example of a photo I captured at sunrise, specifically choosing to use the Tamron 15-30mm lens because it has nine aperture blades, and therefore would provide a starburst effect with eighteen light rays emanating from the light source (in this case the sun).
You can see the photo in question, showing 18 light rays around the sun, on my Instagram feed here:
https://www.instagram.com/p/36alRoJ-fH/
Another reason to consider the number of blades (and shape) of the lens aperture is the shape of bokeh effects and lens flare. Many photographers prefer a very smooth, rounded bokeh effect for example, which calls for rounded rather than straight lens aperture blade edges. More aperture blades can also result in a smoother lens aperture curvature shape, resulting in smoother bokeh.
So, while the number of blades that makes up the lens aperture in a given lens may seem like a rather esoteric specification, there are certainly scenarios in photography where you may want to consider the details of the aperture configuration for a lens.