Perspective and focal length

Perspective in an image is all about the relationships between objects in the scene; specifically their relative sizes and positions and the spaces between them. Photographers can manipulate perspective in several ways to change the impression of space and depth within the scene and provide a sense of scale within it.

Many photographers have the impression that wide-angle lenses produce a different perspective from, say, telephoto lenses. But, in fact, lens focal length has no influence upon perspective as such; instead the perspective changes as the camera position or viewpoint changes.

Changing the lens focal length without changing the camera position has no impact upon the perspective in the scene, even though it radically alters the size of objects within the scene, as the illustrations below show.

 

However, if you keep the size of the main subject the same and change your shooting position as you swap from one focal length to another, the perspective changes. So, too, do the relationships between different elements in the scene, as illustrated in the sequence of shots reproduced here.

Perspective Distortion

The pictures taken from different camera positions reveal the extent to which the camera position and lens focal length combine to distort perspective. This happens because as the angle of view of the captured image varies, so do the relative distances within the scene.

Perspective distortion assumes two forms: extension distortion, which is associated with wide-angle  lenses; and compression distortion, which is produced by telephoto lenses. In both cases the degree of distortion is related directly to the angle of view of the lens.

When photographs are taken from close distances with a wide-angle lens, objects close to the lens will appear abnormally large relative to more distant objects. Distant objects will also appear abnormally small and, therefore, more distant. The impression is that distances are extended.

 

To illustrate perspective distortion, look at the image above. The first photo was taken with a 28mm wide-angle lens, and the second from much the same position with a 300mm telephoto lens.

Perspective distortion depends upon the distance between the subject and the camera. Wide-angle lenses tend to be used closer to subjects because they have a wider field of view, while telephoto lenses have a narrower field of view and are generally used from further away.

To take an example: suppose you were taking a portrait shot and had a choice of three lenses: a 28mm wide-angle, an 80mm short telephoto and a 300mm long telephoto. If you were standing at a distance that would allow the 80mm lens to capture the subject’s head and shoulders, the wide-angle lens would probably encompass most of the subject’s body, while the telephoto lens may only capture the eyes and part of the nose.

However, if the three lenses were used at distances that would allow the photographer to fill the frame with the subject’s head and shoulders, the wide-angle will be used from closer, making the nose larger with respect to the rest of the face, while the telephoto will be used from a much greater distance, making the nose appear smaller.

Perspective distortion is also influenced by the angle of view at which the image is captured by the camera. Wide-angle lenses are particularly sensitive to the angle-of view. Tilt the camera slightly upwards and all the angles in the subject change quite dramatically. Compare the image taken with the 24mm lens with others in the series of shots taken to show the tower at the same height using different focal lengths. You can see the degree to which the verticals have been tilted and how much more of the interior of the tower is revealed with wider angles of view.

Using Perspective Distortion

Perspective distortion can be used by photographers to create certain impressions. Wide-angle lenses can emphasise a particular element in a scene by making it appear larger and spatially removed from other elements.

If the camera isn’t perpendicular to the subject, parallel lines converge more than with a normal lens due to the wider total field. When the camera is pointed upward from ground level, buildings appear to be falling backwards because more of the subject building is visible in the wide-angle shot.

Extreme wide-angle lenses ““ particularly fish-eye lenses ““  can introduce so much extension distortion ( barrel distortion) that they make the resulting image grotesque and disconcerting. The effect increases as the camera is moved closer to the subject, while the degree of distortion depends upon how short the focal length is for the same field size.

In contrast, telephoto lenses are often used to create an impression of compressed distance between objects, which reduces the apparent depth in the subject. This ‘flat’ perspective can be useful for landscape shots, where you wish to suggest different parts of a scene ‘belong’ together. It can also be used to convey feelings of crowding and congestion.

Perspective and Viewing Distance

People judge distance by the way elements within a scene start to look smaller and the angle at which lines and planes converge. For a ‘normal’ perspective, the recommended viewing distance for images is approximately equal to the image diagonal.

However, this will show the distortion effects created by the angle of view of the capture.

Viewing wide-angle pictures from a closer distance will widen the angle of view, reducing the apparent distortion. Similarly, viewing pictures  taken with telephoto lenses from a greater distance, which narrows the angle of view, will reduce the compression effect. In both cases, there will be a critical distance at which the apparent distortion disappears completely.