Steve Cushing Impresionist Fine Art Photography

Embracing imperfection, recording emotions, one impression at a time…

What is Perspective?


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Most people understand the basic notion perspective.An object gets smaller and smaller as it moves away until it disappears at a vanishing point. This aspect is used in drawing and painting to good effect. The term perspective literally means ‘to look through’, and an accurate record of the spatial relationships of objects in a scene is given by placing a glass plate, the picture plane, in the observer’s field of view and tracing the necessary outlines. A painting or drawing may appear to follow exact rules of perspective as obeyed by the technical draughtsman, but usually true perspec­tive is only used for the main aspects of the subject, and detail is subtly altered to give a more pleasing perspective.

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But a camera is not a drawing. A camera usually records a three-dimensional scene as a two-dimensional planar image. The convergent rays form a central perspective and continue divergent beyond the viewpoint to an image plane which is conjugate to the subject plane.
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The subject plane is a flat plane on which an image is projected from a lens which has to create the flat plane. Of course a pinhole camera would give true perspective, but one that is limited by focus and blur. The pinhole camera produces a true perspective in that the small aperture forms a viewpoint for convergence. The centre of perspective of a camera lens is the centre of the entrance pupil for object space and the exit pupil for projection into image space. In order for the principal rays in both spaces to be parallel, the pupil and nodal points should coincide.


Different focal lengths act in the same way as a zoom. They move the sensor relative distance from the lens plane. By altering the lens to one of longer length a bigger image is given but of identical perspective, assuming that both lenses are distortion-free. Altering focal length at a given viewpoint does not therefore alter perspective, only image size.

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So far so good, but in projecting an object on a flat plane we would get distortion. Imaging shinning a light on a flat surface and then changing the angle of the light to the surface. The diameter of the beam will change to an ellipse. The actual beam stays the same diameter but its representation does not. The use of a very wide-angle lens with field of view up to 120° can give rise to an unpleasant perspective. But the shape distortions often observed at the image often incorrectly described as perspective distortions. In pictorial terms, a near viewpoint gives a ‘steep’ perspective with large changes of scale in a subject, while a distant viewpoint gives a ‘flattened’ perspective with only small changes of scale even in a subject of some depth.

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It is not only this distortion that need to be compensated for in the lens design as even for two objects of the same size, with the nearer one distance "a" and the other one at "b", the images perspective size will differ less as the distance becomes greater. For objects at distances x and y, the nearer appears twice as large as the further, but at distances 10m and 11m respectively, their difference is 10 per cent which is noticeable, get further away and this becomes a real issue. But for the average head with a distance of 75 mm from the ears to the tip of the nose, a viewpoint at 2 m gives the nose on a scale only 5 per cent larger appearance than the ears, which is acceptable. Small children are exceptional in this rule.


For more information read sections on Optical Design, Glass, Chromatic Errors, Perspective, Lens Flare and Vignetting. Click on item to go to page.

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