I invested some time today and measured the spectral transmissions of some 135mm lenses I had around.
[click on graph gives a larger image]
Top to bottom:
1) Pentax Takumar f2.5 135mm, multicoated (white)
2) X135 f2.8 135mm calibrated for UV lens, coated (turquise)
3) Enna f3.5 135mm lens from the 50ies, single coated (violet)
4) modern M42 non-name japanese f2.8 135mm lens, multicoated (orange)
5) Steinheil Quinar f3.8 135mm lens, multicoated (blue)
[please ignore the dips and jitter of the measured curves, that comes from that setup/Deuterium lamp used, this can also bee seen on that red 100% line w/o lens...]
I took the 365nm line as the wavelength to decide about the usebility of such a lens, since it is the center transmission of that famouns 2" Baader U-filter I mostly use for UV photography. The X135 @365nm reaches 73% UV transmission (and it transmits down to about 320nm which correlates fine with the end of the UV sensitivity of an UV sensitive camera such as the Nikon D790 or D70s), wheras the next best one has only 39% (Quinar). Lens No.1 is a good example of what could be expected from a modern, multicoated lens in terms of UV transmission - not much actually since the multicoating and internal cemented lenses efficiently block UV, thus leaving such a lens quite useless for UV photography as compared to #2!
Please bear in mind that a useful UV transmission is just ONE criteria (but an important one) for successful UV photography. Resolution, sharpness, contrast, stray light and flare control, lack of hotspots etc. and focus shift of course are other important factors to consider.
HERE is a list of all lenses good for UV photography.
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