Originally posted by Jonathan Mac I think it's possible to open them up and add a smaller aperture (perhaps with black card) to effectively stop them down, reducing light but improving IQ. Could be worth looking into.
The last time I had one of these lenses I tried putting a stop behind it, roughly where it would be in the original Pentax 110 - one of the oddities of the design was that the mirror, shutter and aperture were all combined, I think it worked a little like the weird cylindrical shutter on early Exa cameras, but with aperture control managed by varying the width of the shutter opening, or something of the sort. My stop didn't work very well, not much improvement plus there was vignetting at the corners, but that was probably because the adapter I had then was a DIY effort and not very good. If I can get hold of a small iris assembly from a cine lens or something I might investigate the possibility of mounting it in the Pixco adapter I have now. I suspect it would be a swine to adjust though.
i've added pictures of the gear I was using to my flickr album - both cameras, the adapter, one of the teeny lenses on my camera, etc.