The first idea which might have caused softness was IR leakage, but since that special XBV2 filter stack used includes an IR suppressing filter, this was not the reason.
Since I have an alternative filter stack called XBV2a, I tested it against it - with a a big surprise: A much sharper result could be obtained.
All shot using UV-Rodagon 105mm at f8 using Xenon high Power flash and otherwise identical conditions except the filters used.
[click on image to see a larger one]
XBV2 (left) versus XBV2a (right):
It gets very obvious, that the use of the XBV2a filter results in much improved sharpness, whereas the XBV2 filter result looks rather soft (1:1 pixel presentation).
Here now the two images full size:XBV2 (old version):
XBV2a (new version):
Well, I'm not yet completely done finding out the reason for the softness of the XBV2 filter stack, so...
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