Well, I recently bought that CERCO f4.1/94mm UV-VIS-NIR lens made by SODERN in France (type 2085). It is no longer available on stock (but on special order from 10 onwards) and has been superseded by the faster 2.8/100mm type 2178. Manufacturer site is linked here.
Here a few tests I made with this surprisingly little, but well made lens. It has a built in double helicoid, which works very smoothly and makes close focusing possible. The front filter thread is non-standard, so I have to have an adapter made for it later. Indeed the rear Nikon-F mount does not allow to attach it to a Nikon camera for infinity shooting, as it protrudes to deep inside. For closer shots it works however or used via adapter on different cameras.
So here a few test results. First the transmission of that lens, which is surprisingly much better than expected and also what is given by the manufacturer. [Measuring the transmission of lenses is not trivial and it took me a while to get that right...]
[click on image to see a larger one]
Here the spectral transmission, also with Baader-U filter attached to it (and Baader-U filter alone)
Sodern claims that they have applied an optimised multicoating for 220-480nm which indeed has a positive impact and gains more than 10% in transmission in the UV range. [please note that my spectral measurements are reliable from about 310nm onwards]
Now some photographic results ...
Tests have been done on a dull and overcast day, no direct sun. At infinity shot wide open, it lacks quite some contrast, but shows quite some high resolution. Stopping down (it has no click stops nor markings), contrast enhances as does resolution - BUT stopping down too much produces a very prominent VIS hotspot. I haven't located that effect in UV strangely enough.
VIS stopped down (ca f8):
VIS stopped down full (ca f16) - hotspot:
UV using Baader-U filter, stopped down (ca f8):
Differential VIS-UV shows very little focus shift:
Comparison VIS (left) and UV (right) in bw (gamma/contrast adjusted):
and the same zoomed in:
That lens has obviously not been designed for infinity work, but for closeup.
Under controlled lighting conditions, that lens is a decent performer and shows no hotspot issues for closeup work (until now).
VIS stopped down (ca f5.6):
UV using Baader-U filter, stopped down (ca f5.6):
Differential VIS-UV (shows little shift, but that was me holding the UV filter!)
Images straight from the camera, uncropped. Quite nice I have to say...
More testing has to be done of course...
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