Converging verticals.
The problem of parallel vertical lines appearing to converge in certain types of photography; most notably architectural photography.
If you stand on the ground, tilt your camera upwards and take a photograph of a tall building you’ll notice that the building appears to be tilting backwards, as if the upper portion were leaning inwards. This effect, known as keystoning, is caused by the angle at which you’re viewing the structure.
There are two ways of dealing with this problem. First, you could angle the lens level with the horizon and take a photo of the building from further away. Then you could crop out the unwanted bottom section of the frame. However, this technique is clearly unsatisfactory much of the time - many locations won’t let you step further back from the subject (cliffs, streets, etc), and cropping results in a loss of image area.
The alternative is to use a shift or perspective control lens which can correct for this type of problem by shifting the lens - moving the lens axis (its optical centre) away from the centre of the image area. Or use a camera with bellows which is capable of lens movements.
cf. bellows, keystoning, movements, perspective control lens, tilt-shift lens.
Entry last updated 2002-04-05. Term 274 of 1487.
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