Schneider Componon Lens Puzzlement

I reciently acquired two Schnieder Componon enlarging lenses, one a 50mm F/4, the other an 80mm F/5.6, with a view to using them on a macro bellows. These lenses are tiny things, I think the mounting thread is only 25 mm. They have 15 blade apertures, and I marveled at the skill of the person who assembled these, my own attempt at reassembling a much larger 7 blade aperture on an old folding camera having ended in fumbling humiliation.

Now here's the question. Having looked at them more carefully I realised there were actually two sets of 15 blades in each lens, one on top of the other. Both sets open and close as you change the F number, but only the set towards the front of the lens actually controls the aperture. The set to the rear always remain more widely open than the rear set, and so do not do anything at all to the passage of light. It's a triumph of miniature engineering, but why on earth was the aperture made like this?

 

The attached picture hopefully shows what is meant. Any comments would be appreciated.

Well, I never... I've just checked my Componon 105/5.6 enlarging lens, John, and it's of the same construction, something I'd not noticed before. A search on the Net has turned up nothing, as I'm sure you've discovered. I'm familiar with dual iris construction in the STF (Smooth Transition Focus) lenses, but that's a completely different concept. I can only think there must be a mechanical reason for this; possibly the larger rear blades are linked to the aperture ring and move the smaller front blades in some sort of tandem linkage, simplifying the process of moving such a large number of tiny blades. Beautiful engineering, as you've observed, and I hope someone can come up with an explanation.

The blades are probably L-shaped, and the two rings are really part of the same blades.

I bet John Shriver is right. I seem to recall one or two Soviet lenses that also look like they have two sets of blades.

I don't think so...The two sets of blades are on different levels and move seemingly independently...To be L-shaped wouldn't they have to be on the same plane?

I just found a discussion on the Componon 100/5.6 that describes the construction. It sounds like a scissors-like configuration:

 

https://www.photography-forums.com/threads/schneider-componon-s-100mm-f5-6-double-iris.91741/

Yes, Dave, I'd looked at that and it seemed to be heading in the right direction, but it talks only of a five-blade construction, not the 2x fifteen blade appearance we have here. From the front it looks like an ordinary 15 blade iris. One would think that, with a scissor-like construction, there would be some evidence of overlapping between the smaller "blades" visible in the front and the larger ones at the back, but there's not.They just look like two sets of blades on different planes. Edited March 14, 2017 by rick_drawbridge

What does the front set of blades look like from the front, when stopped down to a small aperture? I ask because I found a patent by Deckel (makers of Compur shutters) for an iris with two sets of blades. One set forms the aperture, but, for the sake of compactness, fails to cover its own periphery completely when stopped down. The second set of blades is for that. The two sets of blades are on the same pivot pins and are closed by the same adjusting ring.

Here's the US patent: https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2949076A&KC=A&FT=D&ND=3&date=19600816&DB=&locale=en_EP#

and the British one: https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=804589A&KC=A&FT=D&ND=3&date=19581119&DB=&locale=en_EP#

both at the European Patent Office site.

Well, here's a shot of the blades from the front, at minimum aperture. I've had to unscrew the lens cells to get these pictures. It doesn't look on the face of it as if there are any gaps, requiring closing by a second set of blades.

 

The whole aperture assembly is only about an inch in diameter.