![]() ![]() the use of special software on the projector.moving the projector more to the center of the screen,.The problem arises for screen projectors that don't have the depth of focus necessary to keep all lines (from top to bottom) focused at the same time. This is particularly noticeable when the lenses are close to the subject, as with a stereo microscope, but is also a common problem with many 3D stereo camera lenses. If the two images are not exactly parallel, this causes a keystone effect. ![]() ![]() In stereoscopy, two lenses are used to view the same subject image, each from a slightly different perspective, allowing a three-dimensional view of the subject. The distortion (on a two-dimensional model, and for small focus angles) is best approximated by:Ĭos ( ε − α 2 ) cos ( ε + α 2 ) is zero, or perpendicular to the screen. The distortion suffered by the image depends on the angle of the projector to the screen, and the beam angle. The effect is usually corrected by either using special lenses in tilt–shift photography or in post-processing using modern image editing software.īrooklyn Bridge keystoning Theory Likewise, when taking photos looking down, e.g., from a skyscraper, buildings appear to get broader towards the top. In photography, the term is used to describe the apparent leaning of buildings towards the vertical centerline of the photo when shooting upwards, a common effect in architectural photography. Some areas of the screen may not be focused correctly as the projector lens is focused at the average distance only. 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. It is the 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. The keystone effect is the apparent distortion of an image caused by projecting it onto an angled surface. Comparison between an image which hasn't been keystone-corrected (top), and one which has been keystone-corrected (bottom). ![]()
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