Tuesday, January 18, 2011

A simple tutorial for reflected UV photography II

I have been asked to write down a simple "cook book style" tutorial on UV photography, which I did quite some time ago. I also wrote about the system lens + filter for UV. But meanwhile some things have changed, more research has been done, so I thought it would be time for a version II. So here it is, based on my personal experiences and likes.  [last update: 2012-09-16]

1) get a suitable lens for UV: I made a comprehensive list of lenses tested good for UV here. The EL-Nikkor enlarger lens 5.6/80mm has proven to not only have the highest UV transmission, but also is virtually free of focus shift. That and all enlarger lenses without focusing ability additionally need a focusing helicoid. Alternatively get the Novoflex Noflexar 3.5/35mm lens which also has a very high UV transmission, is virtually free of focus shift and allows close-up shots up to 1:2 (0.5x) without additional tubes or helicoids. Both lenses are ($)$$, easy to find and may be suitably mounted to achieve infinity focus. If you need a wide angle lens, that's a bit more difficult, but there are some. Still the best and the "industry and scientific standard UV lens" is the discontinued Nikon UV Nikkor 4.5/105mm, or the identical, still available from stock Tochigi Nikon UV 4.5/105mm. Also still available from stock are the JENOPTIK Coastal Optics 4/105mm UV-Micro Apo and 4/60mm UV-VIS-IR Apo Macro but those four lenses are a quite costly $$$$ investment and only fit Nikon cameras directly.

2) get a suitable filter: The 2" Baader U-filter still is the best filter you can get, better than 80% peak and 325...395nm transmission band, IR excellently suppressed, which is important for most DSLRs due to their high IR and low UV sensitivity, especially if your camera has the internal filter removed. Be prepared to expose 8-10 stops more than normal. My usual setting on an sunny to overcast day is 2...4" @ f8-11 ISO200 using a Nikon D70 or converted D200. Best is to have that filter built into a Nikon AF-1 gel filter holder, which needs a 60mm (male) - 48mm(female) and 48mm (male) - 52mm (female) step ring to mount the filter inside and also allows to mount a 52mm sunshade outside. That contraption allows to flip down the filter, adjust focus, take a VIS shot if needed for comparison studies, flip up and take a UV shot. But also screwing in and out of the UV filter works reasonably well if done with care - I do that all the time. Should you have a full spectrum converted camera (with clear filter inside), an external filter which compensates that is needed, so as to be able to shoot normal visible photos with it (as before the conversion). I recommend to use an external 2mm thick SCHOTT BG40 or SCHOTT BG39 or  Schott BG38 (in that sequence of usefulness) for that purpose and adjust white balance accordingly. Some companies offer such "compensating filters", but basically it is just the filter glass I have mentioned.

3) get a suitable UV enabled camera: My finding is that the Nikon D70(s) is the best value for money for UV as is the D40, since both work off the shelf unmodified. D80 and D200 work well either, but need the internal filter exchanged against a clear quartz glass filter first (use a professional service for that: maxmax and lifepixel in the US, Optik Makario in Germany for instance). CANON shooters - it does NOT work with Canon DSLRs, their filters and CMOS chips do not allow to record UV in an acceptable manner (see the test I have published here; extremely long exposure, high ISO, noisy results). Some Pentax, Sony , Panasonic and Olympus DSLRs also work, but all need to be converted first. Recently the Nikon D7000 has turned out to be a useful camera for UV, but needs a converting company, that can deal with the internal IR leakage caused by Nikon's IR shutter monitor (I have reported about that here in detail).

4) get a suitable UV light source: the sun is of course the best, but not always available. Xenon flashes come next as they have a very similar spectrum than the sun has. Most need the front filters removed, some even need the golden UV suppressing coating on the Xenon tube partially (leave a bit on the back or it does not ignite anymore!!) polished off (using fine Cerium oxide powder). A well known candidate is the good and cheap Vivitar 285HV flash (use 2 or 3 if needed), as it already has a clear tube and a low trigger voltage which is safe for modern DSLRs. Another one would be the stronger Nikon SB-14 hammer head flash, but here the tube needs polishing for higher UV output. Another alternative are modern UV LEDs, here especially the Nichia 365nm types. These are available nowadays built into torches, so are easy to use and not that expensive anymore. There are also more advanced systems available of course, which even use the most recent 4 dice high power UV LEDs.

5) get a sturdy tripod: you need usually at least 2...4" exposure time in full sun outside, sometimes much longer though, which calls for a sturdy setup on a tripod. UV is strongest 90 degrees to the sun where the sky is the bluest; avoid to shoot in bright midday sun, due to high IR content present (although no longer a big problem with the new Baader 2" U-Filter, but still IR leakage may happen). Using Xenon flash brings exposure time down to 1/200, but the flash need to be strong and quite close to the subject, which is not always doable.

6) get a suitable shooting habit: for comparison shots I shoot visual light first and then flip up the UV filter and shoot UV with exactly the same framing. Be careful not to move the camera. Should you be using using an older lens which most likely has focus shift, focus closer. This needs to be tested out, my finding is that the f8...f11 position on the DOF scale works best (similar to an IR mark on some lenses) but could also calibrated for that lens (like a IR mark but for UV). Shoot RAW files or high resolution JPEGs. Pro's do RAW, but for first tests JPEG will do. Set camera white balance to 2500 degrees Kelvin to avoid a blown red channel (Nikon only) or use UNI white balance.

7) get the results processed: Upload pictures to your PC and process them. They will look very red (if using Nikon DSLRs, other DSLRS produce blue or magenta looking results), but what you see is UV, depending on camera (and only if a good UV filter with no IR leakage has been used, like the Baader-U) . Then either you process them to black/white or white balance them. Then adjust to taste. UV has no "color" by definition, so you may set what you prefer. For critical pictures I use BibblePro and use its "click white" operation and the built in denoisifier. Others are more happy to use Nikon NX2.

8) get help and discuss results: exchange and discuss your results on one of the few UV forums on the net,  nikonians.com (where I am a moderator), nikoncafe.com and openphotographyforums.com. Quite a bit of my old content resides on the Nikon-only forum  nikongear.com but I am no longer contributing there.

This is in condensed form the result of some years of research and testing and of course only reflects my personal opinion - others may think differently. No warranty given or implied for all being mentioned here and you take full responsibility for all you do with that. If you need some advice or equipment, let me know, I may be able to help. I have plenty of filters, lenses etc. available, since I tested so much for the last years or may be able to direct to a source.

All my works shown here on my BLOG, my pbase.com site or my macrolenses.de site carry my copyright, so if you like to use any of my graphs or my pictures, please do ask me for my explicit permission first.

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site www.pbase.com/kds315/uv_photos