Clear & Ultraviolet

Clear filters, also known as window glass filters or optical flats, are completely transparent, and (ideally) perform no filtering of incoming light at all. The only use of a clear filter is to protect the front of a lens.

UV filters are used to reduce haziness created by ultraviolet light. A UV filter is mostly transparent to visible light, and can be left on the lens for nearly all shots. UV filters are often used for lens protection, much like clear filters. A strong UV filter, such as a Haze-2A or UV17, cuts off some visible light in the violet part of the spectrum, and so has a pale yellow color; these strong filters are more effective at cutting haze, and can reduce purple fringing in digital cameras. Strong UV filters are also sometimes used for warming color photos taken in shade with daylight-type film.

While in certain cases (such as harsh environments) a protection filter may be necessary, there are also downsides to this practice. Arguments for and against use of protection filters incude:

For:

• If the lens is dropped, the filter may well suffer scratches or breakage instead of the front lens element.
  
  
• One can clean the filter frequently without having to worry about damaging the lens coatings; a filter scratched by cleaning is much less expensive to replace than a lens.

Against:

• Adding another element degrades image quality due to aberration and flare.
  
  
• It may reduce the use of lens hoods, since threading a lens hood on top of the clear filter might cause vignetting on some lenses, and since not all clear filters would even have threads allowing a hood to be attached.

Additionally, users of UV filters must be careful about the quality of such filters. There is a wide variance in the performance of these filters with respect to their ability to block UV light. Also in lower quality filters, problems with autofocus and image degradation have been noted.