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Uses of Iris Blades ?


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Apart from shape of bokeh, what are the pros and cons of having more iris blades ?

Cooke S7i (image circle 46.3 mm)         - 9 blades

Zeiss CP3 (image circle 46.3 mm)         - 15 blades

Zeiss Supreme (image circle 46.3 mm) - 16 blades

Arri Signature (image circle 44.7 mm)   - 11 blades

Leica Thalia (image circle : 60 mm)         - 15 blades


Edited by Ram Nanda
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An even number of blades produces the same number of points in a star burst when pointed at the sun or other bright light; an odd number produces twice as many points as the number of blades.
But the shape of the bokeh is the primary reason for having more blades.

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Having rebuilt dozens of iris assemblies over the years I find it fascinating how many different iris designs there are. I believe the choice of blade number and shape often has to do with trends in cinematography or photography, but sometimes it’s just economics or form/function.

Many older lenses have a large number of simple curved blades. These create very round apertures all the way through their range, and the large number of blades also means they can be narrower and thus take up less space in the area outside the aperture. This is important for large apertures in small barrelled lenses, but the downside of these designs is that the aperture closes at a logarithmic rate, so that the smallest f stops end up all squashed together. 

When the blade shape is altered to more of an L shape, the aperture marks can be better spaced, so modern lenses generally have evenly spaced aperture marks with more complicated blade shapes. These tend to take up more space, and so fewer blades are actually required to cover the aperture. 

There have always been lenses with fewer aperture blades, often cheaper brands did this to cut costs, but during the 60s and 70s when lens flares and starbursts came into fashion a lot of expensive lenses were made with 5-7 blades, or even triangular apertures like the early Zeiss Super Speeds. A more polygonal aperture tends to produce better starbursts than a round one, so there were a lot of pentagons, hexagons and heptagons. I suspect auto-aperture stills lenses also benefitted from fewer blades in the iris. 

Even into the 90s and 00s, cine lens makers like Zeiss, Cooke and Angenieux were still producing lenses with a fairly small number of iris blades and polygonal apertures. Over the many years of their production Cooke lenses have gone from many blades, to few blades and now back to many, but they almost always used an iris blade shape that created apertures that went from circle to star to polygon. I don’t know how much that affects the “Cooke look”, or whether it helps to create starbursts, but it’s an interesting continuity.

Since bokeh has become such a talked about aspect of the photographic image in the last 20 years, lens makers have proceeded to add more blades to their irises, to the point where 11 to 19 blades are becoming the norm. These create very round apertures, but depending on the blade shape they can also morph the aperture into star shapes or pointed polygons as they are stopped down, allowing for rounded bokeh at wider apertures and starbursts at smaller ones.

I’d love to talk to a lens designer to ask if there are any other design reasons for their choice in iris blade number and shape. There are so many varieties.

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Several years ago I was looking for more information about why Zeiss might use a triangular aperture (technically a Reuleaux triangle) and came across a white paper I wish I'd saved about the effect of aperture shape on laser optics.  

What I recall from it is that a perfectly round aperture produces concentric rings of diffraction, radiating outward evenly in all directions; while a triangular aperture has lines of diffraction that radiate away parallel to each pair of corners, kind of like a 6-point star filter, with areas in between the lines unaffected by diffraction.  So my guess is that Zeiss was trying to delay the onset of diffraction-related softness at small f-stops, to keep the image acceptably sharp for a wider range of apertures.  

It certainly seem characteristic for Zeiss at that time to care more about the sharpness of the in-focus parts of the image than they would about the softness of the out-of-focus areas.  

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