It turned out that it seems to be a quite useful lens for VIS and now also IR on my UV camera(s), although it has quite some focus shift also in IR (expected from that simple design). The focal length has been confirmed by further measurements to be around 25 - 26mm, so let's stay with 26mm.
Here is a technical article of my friend Marco Cavina, that covers such ultra wide angles nicely with lots of images and diagrams as well as showing never finished prototypes.
Transmission:
[note: measured at f11]
Now more photographic results.... [click on image to see a larger one]
VIS/IR:
Detail:
Quite acceptable also for IR I'd say, even if there is some loss in sharpness and contrast...
Stay tuned, more will follow on that fascinating subject...
More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos