Cooke S4 rounded iris blades

[Kar Wai Ng]

Is it possible to replace the octagonal-shaped iris blades in Cooke S4's with rounded iris blades? Has any lens technician ever tried this modification?

 

I've heard of iris blades being replaced on other lenses when being rehoused, so at least it seems feasible on a technical level.

[Stephen Williams]

Is it possible to replace the octagonal-shaped iris blades in Cooke S4's with rounded iris blades? Has any lens technician ever tried this modification?

 

I've heard of iris blades being replaced on other lenses when being rehoused, so at least it seems feasible on a technical level.

 

Anything is possible if you have the budget, I doubt anyone would bother, shooting wide open gives you round bokeh!

[Dom Jaeger]

I think you'd need to redesign the whole iris assembly to match the unusually long throw on Cooke S4 iris rings. The S4 iris is designed to go from octagonal when stopped right down, to almost star shaped at 2 stops down, to round when fully open. The individual blades are shaped with a right angle cutout, and open more slowly than other modern cine lenses. An S4 iris throw is about 100 degrees compared to say a Master Prime which completes its range in about 65.

 

It's interesting that Cooke favour this iris shape. Other top manufacturers (Zeiss, Angenieux, Canon) all prefer a more rounded, stable aperture, at least in their modern cine lenses. I've seen star shaped apertures in older lenses, like certain Taylor-Hobsons, early Zeiss Standards, spherical Lomos or Kodak Cine Ektars, and they often follow the same aperture transformation, with a pronounced star shape 2 or 3 stops down. Apart from bright, out-of-focus highlights, I'm not sure exactly how much the shape determines the bokeh. The internal aberations of an individual lens also play a part.