(1) Opaque material (usually thin plastic) placed in front of the lens like a filter to block some of the light entering a lens. The mask may have a cut-out shape (a keyhole or heart-shape, for example) or may block half of the image frame to facilitate a double-exposure. (2) Opaque frame used to hold down the edges of photographic paper when making a print. Since the margin area beneath the mask is not printed, the print will have a white border.Photo Tips
An image editing device used to cover part of a digital image that can then be left untouched while you apply a filter or effect to the rest of the picture.
A device placed in front of a lens to reduce the horizontal or vertical size of the frame or to create a particular shape (for example, periscope eyepiece, binoculars, or gun-sight).