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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?

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@j-g-faustus — fixed link. Sorry about that! –  mattdm Apr 16 '12 at 23:17

2 Answers 2

The edge of the blade may reflect light. Such internal reflection is certainly to small to produce visible flare but may introduce some kind of blur. Rounding the blades will reduces this parasit reflection.

As these reflection may possibly show-up in the bockeh it may be slightly improved.

Diffraction will be not be directly affected.

I don't know if the effect of rounding the blade is really visible, however it hopfully indicate that ingenieers have also taken care of other optical defects with the same or higher impact.

Otherwise it's just a marketing argument.

Let me elaborate about diffraction: The wave has to pass through the hole between the blades. Using Huygens-Frensel principle (building the next wavefront by tracing circle with same diameter on each point of the previous wavefront) you can see that it the second side of the blade encountered by the light which determine the exiting wavefront. Then the light will propagate until the sensor where one record the typical interference of the diffraction. Therefore only the side of the blade closer to the sensor account for the diffraction. The blade thickness has no impact.

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This is n contradiction to Stan's answer, which I've accepted along with dim memories of high-school physics class. Can you elaborate on the point about diffraction and edges? Thanks! –  mattdm Apr 16 '12 at 13:12
1  
Academic studies of the diffraction by thick hole are available. See as exemple A.Roberts Electromagnetic theory of diffraction by a circular aperture in a thick, perfectly conducting screen (paper.gentilemathieu.free.fr/Roberts87Elec.pdf) the fig. 7 show the diffraction pattern for a normally incident wave for different thickness of the hole. There is no significant differences. –  floqui Apr 17 '12 at 7:47

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:

enter image description here

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.

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Thanks Stan. Do you know of other lenses with aperture blades made in this fashion? –  mattdm Apr 14 '12 at 15:33
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@mattdm : No, none, and after doing all of the typical water-tank experiments in school and working in audio and radar for a while, I often wondered why not. We did worry about aperture shapes in radar waveguide and antenna design, and it makes an audible difference with speaker cabinets. This is the same thing, but with much shorter wavelengths. –  user2719 Apr 14 '12 at 17:14

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