Поляризационные фильтры для фотоаппаратов

Lens filter for cameras

A polarizing filter or polarising filter (see spelling differences) is a filter that is often placed in front of a camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections (and sky light) tend to be at least partially linearly polarized, a linear polarizer can be used to change the balance of the light in the photograph. The rotational orientation of the filter is adjusted for the preferred artistic effect. For modern cameras, a circular polarizer (CPL) is typically used, which has a linear polarizer that performs the artistic function just described, followed by a quarter wave plate, which further transforms the linearly polarized light into circularly polarized light. The circular polarization avoids problems with autofocus and the light metering sensors in some cameras, which otherwise may not function reliably with only a linear polarizer.

There are two types of polarizing filters readily available, linear and circular, which have exactly the same effect photographically. But the metering and auto focus sensors in certain cameras, including virtually all auto focus single lens reflex cameras (SLRs), will not work properly with linear polarizers because the beam splitters used to split off the light for focusing and metering are polarization dependent. Linearly polarized light may also defeat the action of the anti aliasing filter (low pass filter) on the imaging sensor. Circular polarizing photographic filters consist of a linear polarizer on the front, with a quarter wave plate on the back. The quarter wave plate converts the selected polarization to circularly polarized light inside the camera. This works with all types of cameras, because mirrors and beam splitters split circularly polarized light the same way they split unpolarized light. Linear polarizing filters can be easily distinguished from circular polarizers. In linear polarizing filters, the polarizing effect works (rotate to see differences) regardless of which side of the filter the scene is viewed from. In "circular" polarizing filters, the polarizing effect works when the scene is viewed from the male threaded (back) side of the filter, but does not work when looking through it backwards.

Polarizing filters reduce the light passed through to the film or sensor by about one to three stops (2–8×) depending on how much of the light is polarized at the filter angle selected. Auto exposure cameras will adjust for this by widening the aperture, lengthening the time the shutter is open, and/or increasing the ASA/ISO speed of the camera. Polarizing filters can be used deliberately to reduce available light and allow use of wider apertures to shorten depth of field for certain focus effects. Some companies make adjustable neutral density filters by having two linear polarizing layers. When they are at 90° to each other, they let almost zero light in, admitting more as the angle decreases.