UV-cut filters

As described in the post UVIR-cut filters both absorptive filters and interference filters are sold as UV-blocking filters. All modern photography digital cameras have internal UVIR-cut filters and most modern objectives transmit little UV-radiation. There are some exceptions, most if not all Olympus cameras are sensitive to long-wave UV-A radiation and a few modern objectives also transmit long UV-A radiation. Many filters sold as UV-filters do not differ from those sold as clear protection filters enough to matter. However, a few UV-filters do absorb in the whole UV-band and even into the visible violet band. Their effect might make a difference in high UV environments such as high elevation mountains and snow in sunny weather. On the other hand, most photographers us UV-filters to protect their objectives. For such use the most important considerations are well polished and parallel surfaces and control of reflections to prevent flare and ghosting. Only photography using film and older objectives benefits significantly from the haze-cutting effect of UV and skylight filters.

Based on their transmittance we can guess that the cheap unbranded filter has no anti-reflection coating while the others are all multicoated, with only slight differences in reflectance. The very steep cut-on slope in the top three plots suggests that these filters rely in part on interference filtering to block UV while the bottom three rely on the absorption of UV radiation by the glass itself.

Of the two filters sold as high-quality protector filters the B&W one cuts more UV than three of the UV-cut filters in the figure above, while the Hoya, is similar to some UV-filters from other brands. The antireflection coating in the B&W filter makes this filter almost perfectly transparent to visible light.

Haze filters are difficult to find new nowadays, although they are the only ones that one can expect to have some haze-removing effect on modern digital cameras. They are slightly yellow in colour, especially the stronger types. These are absorptive filters, of which the Haze 2A from Tiffen is a good example. A filter type formerly popular for colour slide film is the Hoya Skylight 1B which does not cut UV radiation effectively, but instead absorbs some blue and green light. With slide film this would correct the bluish cast introduced by UV radiation without actually blocking it. From the plots we can infer that the Tiffen filter is not AR coated while the Hoya is.

Based on a recent post at the KolariVision blog showing the spectral transmittance of the sensor filters removes from various digital cameras plus the spectra for some other UV-cut lens filters, we can conclude that for digital photography the UV-blocking ability of UV-cut filters is rarely relevant. My experience with Olympus cameras suggests they might be an exception if combined with specific objectives.

The optical properties and anti-reflection multi-coating are the properties that will determine whether the filters will degrade image quality or not. With respect to mechanical protection the hardness of the glass and possibly also of the metal in the filter frame (brass vs. aluminun) may be relevant. Some filters have an additional coating that helps repel dirt making them easier to keep clean.

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