Keystoning is a form of geometric distortion that occurs when the optical axis of a projector, camera, or other imaging device is not perfectly perpendicular to the plane of the screen, wall, or subject being imaged. The term derives from the characteristic trapezoidal shape of the distorted image, which resembles the wedge shaped keystone used at the crown of an arch - wider at one edge and progressively narrower at the opposite edge.
In the context of slide and movie projection, keystoning most commonly occurs when a projector is positioned at an angle to the screen rather than directly in front of it and at the same height. If the projector is tilted upward to project onto a screen above it, the top of the projected image will appear wider than the bottom, creating a trapezoidal shape. Similarly, if the projector is offset to one side of the screen, the image will appear wider on the nearer side than on the far side. Because the projected light is travelling a greater distance to reach one part of the screen than another, the image is not only distorted in shape but also uneven in sharpness, as the lens cannot simultaneously focus on surfaces at different distances from it.
Keystoning is also encountered in architectural and interior photography, where pointing the camera upward to capture tall buildings or high ceilinged interiors causes vertical lines to converge towards the top of the frame in a similar trapezoidal distortion. This effect can be corrected optically using a perspective control or tilt-shift lens, which allows the lens plane to be shifted relative to the film or sensor plane to maintain parallel vertical lines, or corrected digitally in post-processing using the lens correction or perspective adjustment tools available in image editing software such as Adobe Photoshop or Lightroom.
In projection applications, keystoning can be minimised by carefully positioning the projector so that its lens axis is as close to perpendicular to the screen surface as possible. Many modern digital projectors also incorporate electronic keystone correction, which digitally adjusts the shape of the projected image to compensate for angular misalignment, though this correction is achieved at the cost of some image resolution and quality compared to a correctly aligned optical setup.