Do rounded edges on aperture blades improve image sharpness, and how?

Question

In the description for the XF lenses for its new X-Pro system, Fujifilm crows:

The [XF lenses] all offer precise control over depth-of-field and deliver excellent out-of-focus bokeh thanks to the design of the molded aperture diaphragm blades. The blades are curved to create a circular image at all aperture settings, while the very edges of each blade are meticulously rounded off rather than simply cut off, which delivers a sharper image. [Emphasis added]

I understand that curved blades — often described as rounded blades — help out-of-focus areas appear more attractive when stopped down, by producing a circular bokeh pattern instead of a polygonal one. But this is bragging about something else: apparently the edges are smooth in the other dimension.

Does this really matter?

• Will it really improve sharpness? Does it affect diffraction in some way?

• What about subjective qualities — might the bokeh be affected in some way?

• Is this construction used on other modern, premium primes with rounded aperture blades, or is it unique?

Asked by mattdm

Answer

When they talk about increased sharpness, they're talking about the blades' edges, not their plan shape. The actual edge of the blade is shaped to be somewhat like a dullish single-bevel knife blade:

That does two things: it means that there is only one edge in play (rather than two, as would be the case with an iris blade that is stamped or otherwise cut with its edges perpendicular to its surface); and that there is virtually no reflective surface in the optical path.

I'm not entirely sure I buy the reflective surfaces part, since that can be overcome with a surface treatment, but the single-diffraction-edge part would be significant, especially at smaller apertures. Now, there's "significance" as a theoretical matter, and there's "significance" in practice, and I'm not equipped to actually test the lens independently of the camera or with differently-shaped iris blades. It may simply be a technical flourish, but Fuji do seem to be going out of their way (between the lack of AA filter, different colour mosaic, etc.) to make a bit of a sharpness and detail statement in the APS-C world, and it just may be the case that the cumulative effect of paying attention to a lot of little details (like the iris blades, the geometry of the exit cone, and so forth) are what they think they have to do to make an appreciable dent in the market.

Answered by Stan Rogers

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