Voigtlander 12mm f5.6 Aperture

That is interesting indeed...

It's all a conspiracy!  

 

Ray, I saw your post and saw no one answered your question. The aperture is correct for the lens--it was never meant to open further (sorry, no conspiracy). You are simply viewing it from the wrong position/angle. This is a 35mm format lens with an angle of view of 115 degrees. If you look at the lens from an angle you will see the edge of the elements line up with the edge of the aperture. Because the field of view is so wide for this lens, the elements need to be large in order for the light to pass through it at that angle. This is why wide lenses have elements larger than the f-number would suggest--I don't see 2mm diameter elements in this design (12mm/f5.6).

What would happen if you did open the aperture further? There would be severe vignetting. Resolution at the edge of the frame would drop off dramatically because of the vignetting, or rather the aperture would become cat's-eye shaped causing a deformed point spread (this already happens at f/5.6 with the Heliar). Most likely, spherical aberration would increase because the elements before and after the aperture are not designed for the greater opening.