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Lenses of shorter focal lengths have wider angles of view. Those with longer focal length have narrower fields of view, and hence bring the subject closer and magnify detail. However, the view they provide of a subject is essentially the same. The image provided by a telephoto lens can be cut from the centre of a picture taken using a wide-angle lens from the same location. However with a lens having a wider field of view than the human eye, distances between objects appear to be stretched out. With a lens having a narrower field of view than the human eye, distances between objects appear to be compressed.

PerspectiveThe size of a subject in the viewfinder can be increased not only by changing the focal length of the lens, but also by moving the camera closer to the subject. However, the images obtained in these two ways are quite different. The one obtained by moving closer to the subject will reveal quite different perspective. In the case of a house photographed from a distance, it may be possible to see a bicycle leaning against a side wall. However from a viewpoint much closer to the house, the bicycle may be concealed around the corner.

Inevitably, the most noticeable perspective changes occur when both the camera position and the focal length of the lens are changed. In such cases, the size of the subject changes, the distances between objects within the field of view may seem to be magnified or compressed, and more or less of the background and the three-dimensional qualities of the subject may be visible.

A camera can not only be moved nearer or further from a subject, but also tilted up and down and swivelled from side to side. Tilting a camera upwards to record a tall building will produce an effect known as converging verticals. The base of the building, being nearer the camera, will appear wider and larger than its more distant top. The vertical sides therefore converge as the building rises and theoretically meet at infinity. This effect is often referred to by photographers as distortion, although this is incorrect. It is merely the correct representation of the physical world from a particular viewpoint. However when the image is reproduced on a convenient sheet of paper which fits well within the normal human field of vision, the perspective seems inappropriate. Converging vertical can be corrected by moving the image plane so it is parallel to the vertical face of the building. Special cameras and so-called shift lenses (perspective control lenses) allow the lens position and image plane to be controlled in a manner which eliminates this problem.


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