Matte or coated filter reducing lens flare and reflections — optical control of unwanted glint without affecting exposure or color.
On set, you'll quickly realize: a shiny front lens is your enemy as soon as the sun hits from the side or you're shooting in front of mirrored glass. Anti-glare filters – meaning frosted or specially coated filters – step in where ND filters and polarizers stop. They don't reduce the amount of light, but rather break up the disturbing surface reflections of the lens itself and prevent the classic, unwanted lens flares.
Practically, this works through two mechanisms: either the filter has a roughened, frosted surface – similar to sandpaper in optical quality – or it has a multi-layer anti-reflection coating. The coating variant is of higher quality because it causes less light loss and doesn't affect image sharpness. The frosted variant has a broader, more diffuse effect but can still produce flares in extreme backlight. Unlike a diffusion filter (see Diffusion), anti-glare filters don't consciously alter the image aesthetic – they are optical hygiene, not a creative decision. You don't notice they are there when they are working correctly.
In everyday shooting, you primarily use anti-glare filters in three scenarios: backlight situations with harsh sunlight (especially sunrise/sunset scenes), interior shots in front of large mirrored glass or reflective facades, and anywhere the camera is looking very flatly at a pane of glass. A good anti-glare filter isn't placed in the matte box like an ND filter – it's usually in the outermost position in front of the lens or directly on the front of the matte system. Some DoPs combine it with a slight diffusion (e.g., 1/4 or 1/8) to further kill the glare without sacrificing contrast.
Warning: Anti-glare filters can produce visible artifacts at extreme zoom ranges (telephoto) – fine structures in the frosting become visible. Therefore, don't use them blindly everywhere. Test before critical takes with standard focal lengths to see if it looks optically good. The best solution remains: position lighting and camera angles to avoid reflections from the outset.