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The keystone effect, also known as the tombstone effect, is caused by attempting to project an image onto a surface at an angle, as with a projector not quite centered onto the screen it is projecting on. It is a distortion of the image dimensions, such as making a square look like a trapezoid, the shape of an architectural keystone, hence the name of the feature. In the typical case of a projector sitting on a table, and looking upwards to the screen, the image is larger at the top than on the bottom. Some areas of the screen may not be focused correctly as the projector lens is focused at the average distance only. == Theory == The distortion suffered by the image depends on the angle of the projector to the screen, and the beam angle. The distortion (on a two-dimensional model, and for small focus angles) is best approximated by: : where is the angle between the screen axis and the central ray from the projector, and is the width of the focus. From the formula, it is clear that there will be no distortion when is zero, or perpendicular to the screen. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「keystone effect」の詳細全文を読む スポンサード リンク
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