I got a loaner lens for testing, a Hamamatsu f3.5/50mm UV lens, made of quartz and fluorite, a 5e/5g design, with obviously uncoated lens elements (which might be the reason for the prominent hotspot that it shows in VIS and UV), as this is some 15-20 year old lens I have been told. It has a c-mount and was made for Hamamatsu's line of high sensitive and intensified UV cameras, hence its image circle is just 16mm as per its specs. I measured the latter and the real image circle is larger (certainly not as well defined), about 25-27mm, so it will not cover APS-C sensors, that require at least 30.1mm.
[click on image to see a larger one]
First thing tested was vignetting, which it shows quite some wide open, if used with a 21mm image sensor.
Vignetting, fully open on a 21mm diameter imager:
Well, I was curious to see how it would perform in VIS and UV, so I took a few shots with it, using for UV my "work horse" UV filter, the Baader-U. This lens has a common 40.5mm filter thread, so mounting filters was easy, just a step ring was needed. Unfortunately the lens front rotates while focusing as do many other lenses. I used sunlight for the outside shots and Xenon flashlights for the studio macro shots, using a Rudbeckia fulgida flower as a target.
Outside shot, VIS, fully open:
Outside shot, UV, fully open:
Outside shot, VIS, stopped down to f8:
Outside shot, UV, stopped down to f8:
Stopping down significantly reduces blurriness that is visible wide open even in the image center as well as some curvature of field and enhances imaged sharpness considerably.
It gets quite obvious, that stopping down the lens (from about f8 onwards) makes visible a quite prominent hotspot that is has in VIS + UV in certain lighting situations:
Outside shot, VIS, stopped down to f11, central hotspot:
Outside shot, UV, stopped down to f11, central hotspot:
Now on to studio macro shots...
Macro shot, UV, stopped down to f11:
Macro shot, UV, stopped down to f11, detail
The detail shows some strange artifacts which could either result from the flower being wet or lacking lens resolution. More research needs to be done on that.
Here is its transmission graph versus some normal and quartz fluorite reference lenses:
[Be reminded that this measuring setup used is valid to about 310-320nm onwards, so both quartz fluorite lenses transmit UV better than shown]
To summarize, this (rather expensive $$$$) Hamamatsu A4869 f3.5/50mm UV lens is certainly not a bad lens. That prominent hotspot it has needs carefully controlled lighting situations to avoid spoiled images, which in the field outside cannot be secured, so it's "hit and miss" unfortunately. So I would rather recommend it for studio work only. Also the lens needs to be stopped down to at least f8 to achieve a flat field and sufficient sharpness. In studio the lens performs much better, as my examples show and a thin c-mount extension ring added gives the needed close up/macro focusing distance, that this lens otherwise not has (closest focusing is 0.6 meters, 2ft). Infinity focus was perfectly adjusted and no re-collimation was needed.
P.S.: It has been confirmed by Hamamatsu staff that this lens is about 15 years old and was designed to match the needs of maximum 2-3 Mpix cameras. Hence the limited resolution and most likely the artifacts are diffraction effects. Further it was mentioned to me that the found hotspot may be the result of uncoated lens elements being used, so stray and reflected light bounces between them and the shiny sensor surface.
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