What are the technical reasons to design lenses with oddly shaped apertures?

Question

Some lenses have oddly shaped apertures, like star or shuriken shaped. It seems to be intentional since making "normal" rounded or polygonal apertures is fairly simple (as I have observed while disassembling the irises of several lenses). What are the technical reasons to make apertures that way?

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I recall reading a comment somewhere that it's done to correct for some lens defect, but have been unable to confirm. Usually, the blades look like they could be extended slightly to avoid the pointy corners, so it's unlikely an alignment issue.

I am not asking about apertures shaped by the photographer artistic effect.

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From a rebranded 35/2.8 manufactured by Tokina.

From a 35/1.7 C-mount lens.

Answer 1

Out of focus objects are convolved with the aperture shape (meaning that each of their spots produced a little image of the aperture, and all those spots are overlayed). In particular that means point light sources create images of the aperture, the size corresponding more or less to how much out of focus the point light source is. So the shape of the aperture determines what happens to out of focus highlights, and also what happens to out of focus outlines of bright areas (basically, you trace the aperture shape around that outline).

A circle approximation converts highlights into circles, and bright rectangles into comparatively sharp outlined rectangles with oval corners. A shape that does not go completely dark from one radius to the next will do a better job of blurring outlines.

Answer 2

Lenses are limited as to their resolving power (ability to separate closely spaced lines/objects). Optical resolving power was investigated by John Strutt 3rd Baron Rayleigh – Astronomer Royal – Novel Prize 1919 – Resolving power = 1392 ÷ f-no. The 1392 example is about the center wavelength. Thus the RP is different for each wavelength under study.

For this example, lens set to f/5.6 the RP = 249 lines per millimeter. Image degrading is due to diffraction which is connected to the circular exit pupil (aperture). "Apodization" is optical jargon that refers to several techniques to improve resolution by decreasing diffraction (which is devastating). Among these, changing the shape of the aperture to a square or oval or serrated are examples.

Answer 3

I would say the primary effect is to manipulate bokeh characteristics/shape, but not necessarily improve them.

A more rectangular aperture could improve image quality by blocking light that does not conform to the image format/area... much like the lens mask/aperture of the Angenieux design lenses, rectangular bellows hoods, and the matte box used for video.

The first image of a shuriken shaped aperture could be considered to have somewhat of the same effect that an apodization filter does, as seen on the Sony/Minolta STF lens design. It would actually be less effective in correcting errors than a more rounded aperture but would allow a little more light (acting as a larger aperture opening with something of a "faded edge").

Answer 4

Inflation is also known as crapification.

The older cine lenses often had 15 leaves and very round apertures as a result. Cost eventually meant 6 leaves were standard for 20 years or so and now we may have two leaves, making a triangle or a square aperture.