Voigtlander 12mm f5.6 Aperture

[OzRay]

I was looking at the aperture of the latest M mount Voigtlander the other day and noticed that it appeared not to open fully. Here's a shot of the aperture from the back of the lens (nice and round by the way):

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As you can see, the aperture appears as if it could open considerably more than it does. If it could open fully, it would increase the speed of the lens substantially by the looks of it. I contacted the supplier just to see if this was normal or whether the aperture was faulty. It appears that this is normal.

So has Cosina Voigtlander deliberately made it so that the aperture won't open fully, in order to controls aberrations etc? I'm curious, as I'd be happy to be able to open the lens further and possibly live with any aberrations that may arise.

Cheers

Ray

 

[Jamus]

That is interesting indeed...

It's all a conspiracy!

 

[Hikari]

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.

 

[OzRay]

There's obviously a valid reason for it, but the only lenses that I have where the aperture doesn't open fully, when looking at the aperture from the rear of the lens, are the CV 12mm, CV15mm and Minolta 90mm f4. Every other CV and Leica lens opens fully, so that you can't see the aperture blades. I was just curious about this difference.

Cheers

Ray