Well, I tested it good for 330nm in my spectrometer and in theory it should have no focus shift (or just a tad)...
[click on image to see a larger one]VIS:
UV (Baader U-filter, 310-390nm):
Long story short - it has hardly any visible focus shift!
Look at these mites and fine webs!! Pretty fine resolution it has.
More about it later ... and sssshhhh, that lens is a secret until then ;)
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