In previous posts on whitebalancing using my ReflectionDisc I warned about generalizing results and stated that it is only valid for that camera, that lens, that filter and that lighting situation. Well, I was curious, how much would change if two parameters were changed: a) using a wide angle lens (with substantial less UV transmission), b) lighting situation
So, using my GH1-UVIR, a Cerco 94mm lens, Baader-U filter and sunlight, I had preset a whitebalance for a similarely lit situation using my Reflection Disc, call that WB1 for now.
The weather today was very overcast and no sun, just about 250 microWatt/cm² UV-A (as measured using a calibrated Minolta UV-A meter).
The lens was changed, from a color corrected, flat transmitting (<300nm) quartz / fluorite lens to a non-color corrected not that much UV transmitting conventional wide angle lens, a 2.8/28mm in that case, which barely reaches about 350nm, to see which impact that would have. A set WB was done for that 28mm wide angle lens, call that WB2.
So what we see here is:
1) top left: previously set sunlight WB used (WB1)
2) top right: correct WB for that lighting situation set (WB2)
3) bottom left: previously set sunlight WB used (WB1), bw version with histogram
4) bottom right: correct WB for that lighting situation set (WB2); bw version with histogram
[click on image to see a larger one]
Here now quite some difference in whitebalancing may be noted; the preset WB1 was quite different from the correct WB2 for that lens (and lighting situation). Major impact had the lens change, not the changed lighting. As may be seen on the histogram, the correctly whitebalanced image is about 1/4 - 1/3 stop less exposed than the incorrectly one (both images were identically exposed at 1/1.6sec, same ISO) , so consequently using the correct WB at shooting time would require a somewhat higher exposure.
SUMMARY: UV whitebalance, for lenses having very different spectral (UV-) transmission, should be correctly set at exposure, not later in post processing and properly exposed (a tad more).
P.S.: I have tested at different exposure values (higher + lower) but the principal result remained the same.
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