Other effects 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. See also Polarized 3D glasses References External links Polarizer another must-have filter Optical filters Polarization (waves) Articles containing video clips