Filter Photography

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 as well.

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.

Also known as: UV Cut Filter

A polarizing filter, used both in color and black and white photography, can be used to darken overly light skies. Because the clouds are relatively unchanged, the contrast between the clouds and the sky is increased. Atmospheric haze and reflected sunlight are also reduced, and in color photographs overall color saturation is increased. Polarizers are often used to deal with situations involving reflections, such as those involving water or glass, including pictures taken through glass windows (this uses the phenomenon of Brewster’s angle).

Polarizers are the type of filter whose use is least affected by digital photography; while effects that may visually resemble the results of a polarizing filter can be simulated with software post-processing, many of the optical properties of polarization control at the time of capture simply cannot be replicated, particularly those involving reflections. The effects of a polarizer on the sky in a color photograph. The picture on the right has the filter. The effects of a polarizer on the sky in a color photograph. The picture on the right has the filter. In the first picture, the polarizer is rotated to minimise the effect, and in the second it is rotated 90° to maximise the effect – almost all reflected sunlight is eliminated. In the first picture, the polarizer is rotated to minimise the effect, and in the second it is rotated 90° to maximise the effect – almost all reflected sunlight is eliminated.

There are two types of polarizing filters. A linear polarizer filter transmits one of two states of linearly polarized light. A circular polarizer (sometimes called a CPL filter) similarly selects a linear state but then converts it to circularly polarized light, by adding a birefringent layer (typically a quarter-wave plate) to the filter after the linear polarizer. The metering and auto-focus sensors in certain cameras, including virtually all SLRs, will not work properly with linear polarizers, both because of the mirror and because of the beam-splitters used to split off the light for focusing and metering. Circular polarizers will work with all types of cameras.

Also known as: Polarizer