The effect where vertical lines, such as the sides of a tall building, appear to lean toward each other in a photo. It happens when you tilt the camera up to fit the whole building in the frame. The sensor is then no longer parallel to the building. Tilting the camera down causes the same effect in the opposite direction.
To avoid it, keep the camera level and step back, or use a shift lens or a view camera. It can also be corrected in editing software.
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