Keystone Distortion

In cinema projection, various forms of image distortion can impact the audience’s perception of picture quality. This paper examines five key types: vertical and horizontal keystone distortion, vertical and horizontal anamorphic stretch, and geometric distortion caused by curved screens. These distortions arise from projector placement, lens specifications, film format, and theater geometry, affecting the shape of projected images rather than their clarity.

Keystone distortion occurs when a projector’s optical axis is not perfectly perpendicular to the screen, producing a trapezoidal image. Anamorphic stretch results from similar misalignment, causing image dimensions to be disproportionally scaled. Curved screens, often employed to improve illumination uniformity and add depth, introduce geometric distortion, typically in the horizontal orientation, producing barrel-shaped effects.

The paper provides step-by-step calculations to quantify these distortions, using examples of projector height, tilt angle, lens focal length, film format, and screen curvature. For instance, a projector tilted downward in a theater can create up to 9.6% vertical keystone distortion and 3.8% vertical anamorphic stretch, while a curved screen may yield approximately -8.3% geometric distortion.

Based on research, testing, and audience perception, tolerances of up to 5% for each type of distortion are suggested, as distortions below this level are generally imperceptible to viewers. The analysis highlights that these effects depend solely on physical geometry and lens parameters, not on viewer position or perspective. This framework aids theater designers, engineers, and projectionists in predicting, evaluating, and minimizing distortion for optimal cinematic experiences.