Dichroic Filters
Dichroic filters, also known as "interference filters", are designed to work with incoming light that is perfectly perpendicular to the filter surface. This is fine when using dichroic filters for telescopes or lasers; in both these cases, the light is collimated - aligned as parallel - before it reaches the filter. However, most UV-photography has the UV-bandpass filter attached to the front of the lens as the first receptor of light. 
Thus the filter is receiving the light as a cone, with most of the light not at normal incidence. When light strikes the dichroic filter at any angle other than ninety degrees, the filter will act to shift the band of light it passes to a different series of wavelengths. This is illustrated in the graph to the right. This sensitivity to the angle of incidence (AOI) of the light results in some ultraviolet light being rejected that should have passed to the sensor... in other words, a false representation of the object being photographed in the ultraviolet is created. This altering of the incoming UV light becomes more apparent when using wide-angle lenses. We recommend use of our Andrea 'U' MK II for those photographers who will regularly use a lens less than 35mm focal length for their UV photography. For macro photography and other close up work, and those requiring long focal lengths, the StraightEdgeU performs superbly, as does the Andrea 'U' MK II.