Some difficult issues of the digital ultraviolet photography: There are several issues concerned to the digital UV photography.

1. (1)The quantity of the UV light in the solar radiation is scarce in comparison with the IR light and reflected UV light from a photogenic object is considered to be far less than IR light.  Moreover, as the transmission efficiency of the UV light through a conventional glass lens and the sensitivity of the sensor of a digital camera are lower than the case for the IR light, overall sensitivity is very low.  In order to use the UV light from an object effectively it is necessary to employ a special lens whose transmission efficiency for the UV light is high.  Such a kind of lens is commercially available but we do not have sufficient choice and, moreover, such a lens is generally rather expensive.
   

2. (2) Characteristics of visible opaque and UV transparent filters will be described later in some detail.  Generally speaking we have only a few choices and these are rather expensive in comparison with visible opaque and IR transparent filters.
   

3. (3) It is often the case that a visible opaque and UV transparent filter transmits the IR light too.  Therefore, even if one would take a UV photograph there may be a possibility that the finished photograph is an IR photograph.
   

4. (4) There is another problem which is in common with the IR photography, that is, focusing the camera is difficult especially when one use an unconverted camera. This is because the quantity of the light which reach the sensor is scarce.  Though in a usual point-and-shoot camera one can see the UV image on the LCD screen of the camera and the AF function works well, usual digital SLR does not have a live view function and AF may not work if the camera is not converted.  Presently, I use unconverted digital SLRs, Olympus E-300 and E-510, for the IR and the UV photography.  Recently, I got an E-620 dSLR converted for the full-spectrum photography, which I describe later.   The E-510 camera has a live view function and one can check a UV image on the LCD screen but as the quantity of the incident light is scarce I cannot focus the camera to take an IR or a UV photograph by using the AF function.  As the last resort I focus the camera for the visible light image in the manual focus mode and then take an IR or a UV photograph by putting an IR transparent or a UV transparent filter in front of the taking lens.  The problem associated with this method is the focal length of a lens depends on the wavelength of the light.   Though usually a lens is designed so that the focal length is nearly constant over the frequency range of the visible light (achromatic lens) outside the range of the visible light this condition is not taken into consideration.  Therefore, if one focus the camera by the visible light but does not correct the focal length appropriately for the feature of the lens and the wavelength of the light, the photograph may become out of focus.