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Aperture Blades
The number of aperture blades is only one factor in bokeh, but a circular aperture is always very helpful because it minimizes unattractive angular shapes, or even the hint of them.
It is not by chance that the standard Zeiss lenses for Nikon and Canon have 9 blades (excellent for 35mm), but that the same optics used in the Zeiss Compact Prime Cine line have fourteen (14) blades. The more the blades, the more circular the aperture, though individual blades can also have a slight curvature as well. The Zeiss Cine lenses are described this way:
The iris opening of the Compact Prime CP.2 lenses is created by 14 high precision blades. It stays consistently round and symmetrical over the entire T-stop range. This translates into natural and pleasing out-of-focus highlights and a smooth bokeh. Together they help create and capture special moments on film.
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- Eases into photographic challenges with an introductory section.
- Covers aspects of digital sensor technology that relate to getting the best image quality.
- Technique section discusses every aspect of making a sharp image handheld or on a tripod.
- Depth of field and how to bypass depth of field limitations via focus stacking.
- Optical aberrations: what they are, what they look like, and what to do about them.
- MTF, field curvature, focus shift: insight into the limitations of lab tests and why imaging performance is far more complex than it appears.
- Optical aberrations: what they are, what they look like, and what to do about them.
- How to test a lens for a “bad sample”.
Intrigued? See Focusing Zeiss DSLR Lenses For Peak Performance, PART ONE: The Challenges, or (one topic of many) field curvature.
Actual photos of Zeiss 100mm f/2 Makro-Planar diaphragm— f/2 through f/22