The common mention of “ultraviolet” in photography is the use of ultraviolet (UV) filters, which specifically block UV light and allow visible light into the camera. This is less important in the digital days since UV is also blocked at the camera sensor. It was more important in the film days when too much UV light could fog the film.
Today I am talking about the opposite of this – a filter that blocks visible light and permits UV. I’ve owned a full-spectrum DSLR for astronomy for a while now but I’ve only really experimented with infrared (IR). Recently I came into a set of “instrument filters” in a charity shop which turned out to be two UV-pass filters and one IR-pass filters. They were not labelled so I’m not sure what the bandpass is although in bright daylight, with the naked eye I can see dim purple objects through the UV filters and dim red objects through the IR filter.
I’ve got a bunch of lenses, old and new that fit my DSLR. However the instrument filters are 2″ square glass and most of my lenses are too big – they would not be covered by a 2″ filter. Eventually I found two lenses that accept 49mm screw-fit filters, so they can be completely covered by a 2″ square glass filter. The lenses are a Pentacon 30mm f/3.5 wide angle and a Paragon 35mm f/3.5 wide angle (also branded as Photax). Both are M42 fit – easily adapted to Canon EF without extra glass. On the DSLR’s crop sensor, they are not quite as wide – in fact the 35mm lens is about a normal lens on APS-C. After a (very) brief experiment I discovered that the Paragon was a bit better at transmitting UV than the Pentacon, so I decided to use that.
Here are my test shots with these filters, taken with both UV filters, the IR filter and finally with no filter. (To me, #nofilter doesn’t mean a selfie without Instagram). In each case I set a custom white balance for that filter.
The results weren’t quite what I was expecting. I expected the UV pictures to be startlingly different from the IR pictures. However, except the sky, the two pictures are very similar. I think what is happening is that the UV filter must be transmitting UV and IR. The IR is both brighter than the UV, and the camera’s sensor is more sensitive to it. In the bright areas, the IR drowns out the UV. In the sky, which is dark in the IR picture, the UV shows through – hence the purple sky. I’ll have to get onto my tame scientist friend and see if I can use his spectrophotometer again.
Regardless of the science, shooting through this UV filter gives an unusual effect. I went for a short walk on Troopers Hill again and came up with these pictures in the evening sun. These have had no editing except for the custom white balance set on the camera and a slight boost to contrast on the computer.