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Once you have acquired a DSLR and a lens that perform acceptably in the near-UV, you are only half-way to
the goal of UV photography. There are a few
choices of UV-pass filters available, like the U-360.
However, all the normally available ones (including the venerable Nikon FF)
let through also substantial amounts of near-IR. Since most DSLRs are far
more sensitive to IR than to UV, and sunlight contains copious amounts of
IR, using these filters with natural illumination means that you are, in
effects, taking IR pictures containing just a small tinge of UV. Some UV
photographers have solved this problem by stacking together UV-pass and
IR-blocking filters (mostly of the hot-mirror type). One problem with this
solution is that many hot-mirror filters are not designed for UV
transmission, so you loose much of it.
I decided to try another way, and to look into the use of filters designed
for astronomic observation. Astronomers often need to isolate narrow
spectral bands and to cut out everything else. In the context of a UV-pass,
cut-everything-else filter, two or three types are repeatedly mentioned. One
is the Baader U filter (also called Venus filter because it
allows the observation of cloud bands on this planet). The older, more
commonly available version of this filter is a
rather ordinary UV-pass filter coated with 20 dielectric layers to cut the
residual IR transmission. It still does transmit quite detectable (also in
UV pictures) amounts of
near-IR (see
http://www.astrosurf.com/pellier/comparuvfilters). More recently, an improved
"Baader U2
filter" was announced. It pursues the same type of IR-blocking by
increasing the number of dielectric layers to one hundred. It is vastly
improved over the old version, but some sources say it still leaks some IR
in the 1000 nm range, which may or may not be of concern for ordinary DSLR
sensors. If you are interested in this type of filter, be aware that the
dielectric coated surface must face toward the subject.
The second type is the Schuler UV filter, which is built in an
entirely different way. It is a rather thick (a couple of mm) sandwich of multiple layers of UV-pass glass
filters with slightly different spectral properties. These layers have
roughly similar UV properties, but their IR-transmission peaks lie at
different wavelengths. Therefore, virtually no IR passes through the stack (see
http://www.astrosurf.com/pellier/comparuvfilters). The Schuler UV filter
is significantly cheaper than the Baader U, and also cheaper than attempting to build your own stack of
multiple UV-pass filters. It may even be cheaper than the traditional
solution of stacking an ordinary UV-pass filter and a hot mirror.
The Schuler UV filter transmits roughly 60% of near-UV, as opposed to 80%
for the Baader U/U2. The spectral properties of the two filters are also not
identical, with the Schuler peaking at 360 nm and transmitting even a little
visible indigo, and the Baader U/U2 extending farther into the UV. However,
these are not very large differences for UV photography, and practical
results may be counterintuitive (e.g., astronomers have reported 30%
brighter images with the Schuler). If you want pure UV, you should use the
Baader. If you are interested in non-monochromatic results, or simply want
to spend less and still obtain good results, you may use the Schuler.
My choice between Baader and Shuler filters was based on the desire
to keep some colour information, and doing so while paying a lower price did
have its attractiveness. Therefore, the Schuler UV filter was my natural
choice. I found no source in the EU and a couple of US-based Internet shops could not ship
it abroad because of commercial agreements, but
Adirondack Astronomy did, and a 48mm
Schuler UV is now sitting in my bag of tricks (mounted on an adapter to use it
as a standard 52mm lens filter, and on a Nikon AF-1 holder to swing it out of
the way when focusing). There is also a 24mm Schuler UV filter available,
which could be useful in macro photography because of its small diameter.
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The pictures at the left, of the same subject, were taken with the
U-360 (top) and Schuler UV filters (bottom). The two
images were too different from each other to post-process in the same way,
so I was forced to process each picture separately to provide the best
visual result possible with the information present in each picture. The
results speak for themselves. While the U-360 is in practice an acceptable
(if expensive) IR-pass filter, the bottom picture is the only one
that can be called a UV image, and the only one that shows the
characteristic UV pattern used by insects to home-in on the centre of the
flower (i.e., the darker areas of the petals near the centre of the flower).
These pictures were taken with a UV fluorescent
tube and an incandescent lamp (for framing and focusing), a
D70s modified for full-spectrum photography
and a UV Rodagon 60mm at f/11 on
ultra-short bellows. Exposure was manual
(1 second with U-360, 8 seconds with Schuler UV).
There is also a Schuler SP 470 astronomical filter that transmits UV, indigo
and blue, roughly in a range between 350 and 470 nm, and apparently has no leak in the IR range. It might
or might not be useful in photography (UV information usually is recorded at
least in part in
the blue channel, so it would superimpose the blue information, probably with no way
to separate them).
If you shoot UV macro photography and are worried that the thickness of the
Schuler UV filter may cause a focus-shift, there are sets of coloured
Schuler filters that include a transparent one designed for parfocality. Use
it to compose and focus, then swap it with the UV filter. So far, I did not
have this problem with reproduction ratios as high as 1:2. |
| U-360 |
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| Schuler UV |
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