The Canon EF 24-105mm f/4 L USM IS lens has rounded aperture blades. What other types of aperture blades are there, and what are the advantages and disadvantages of each of them? Is the aperture blade shape a manufacturing decision?


4 Answers 4


As far as I know there are two types, rounded and straight. There are also diaphragms with many blades and diaphragms with few.

Basically these factors determine the shape of the hole through which light passes, and in turn this is reflected in the shape of the out of focus highlights in the image.

Diaphragms with straight cut blades tend to have fewer blades and leave distinctive polygonal shaped bokeh discs in images. The advantage is simplicity and cost. Older lenses tend to be of this type. Also the shape of the bokeh is an artistic consideration with many people enjoying the character that this type of bokeh lends to an image.

Here is an example of a five blade ever-so-slightly-rounded diaphragm:


Diaphragms with rounded apertures and many blades produce a circular hole, nice round highlights. This costs more to design and engineer, and tends to be found in modern high end lenses as it gives a smoother look to the bokeh. The aperture shape shows up most easily in highlights but it's not just highlights that get projected as the shapes in the image. It's all points of light not in focus so if you have detail and contrast in the background you get get a rough look with straight aperture blades, instead of a sea of tiny circles.

Here is a more rounded 9 blade diaphragm:

Notice that the highlights are still not totally round. It's worth pointing out that even five blade apertures can produce totally circular highlights wide open, as the aperture blades retract leaving only the shape of the lens barrel. So if you see highlights which are perfectly circular like this:

(source: iphotocourse.com)

You know the lens was shot wide open.

  • 1
    Hi, one of your image links is dead. Could you please fix that? Dec 18, 2013 at 20:50

Following @DarkcatStudios answer, since you did not limit your question to the modern SLR, you can see in the linked article that there are apertures that provide a variable diameter, perfectly circular openings:

rotating diaphragm

[yes, this not directly fall under the category of "aperture blades" but for the sake of completeness of the discussion I added this one]

  • 1
    This also applies to modern SLRs if you're using something like a Lensbaby with drop-in magnetic aperture rings.
    – mattdm
    Jun 4, 2012 at 16:55
  • Hey, @mattdm, I didn't know the discussion is extended to toys ;-) ... [to all Lensbaby fans out there, please, no offense... its (bad) humor]
    – ysap
    Jun 4, 2012 at 17:16
  • As long as they're photographic toys :)
    – mattdm
    Jun 4, 2012 at 17:21

Rounded aperture blades require a more complex opening/closing mechanism than straight cut blades. Also more are required in order to create a rounded aperture, hence they are found only on more expensive lenses. It is a cost-based manufacturing decision.

A rounded aperture produces a softer, more natural bokeh (out of focus areas).

See Wikipedia's article on diaphragms in optics for more details.

  • That's not entirely true. You're repeating a modern marketing ploy rather than actual fact. Plenty of cheap consumer film cameras (many Kodak models, Dianas, etc.) have perfectly circular apertures of various sizes. Conversely many expensive lenses (DC Nikkors, Schneider 47mm Super Angulon XL, etc.) have relatively few straight aperture blades and produce arguably fantastic bokeh. My point is that there is no fundamental difference in manufacturing complexity/cost between an aperture with straight blades and one with curved blades.
    – HamishKL
    Jul 25, 2015 at 2:38
  • Having disassembled several aperture mechanisms with rounded, straight, and varying numbers of blades, I have observed that the roundedness of the blades does not increase the complexity of the design. One of the simplest aperture mechanisms I've seen has eight rounded aperture blades. The problem is it's impossible to get a "proper" starburst out of that lens.
    – xiota
    Jan 7, 2019 at 0:24

There are rounded and straight blades. To have circular "bokeh" it is important to have a big amount of blades, as well as rounded blades.

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