Definition: Distortion of shape of projected image seen in a failure to project squarely onto the projection screen i.e. so that the optical axis is perpendicular to the screen e.g. when slide projector is tilted upwards to project onto a wall, image shape looks trapezoidal, i.e. like a keystone. * As the upper part of the image is further away from the projector than the lower edge, the image is projected at a greater magnification: obviously this occurs only when the image and the subject planes are not parallel to each other.

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