An article discussing perspective in a general sense may be found in the Optics section. However, this article is intended to further the discussion from the point of view of image composition.
Perspective is the perceived relative relationship of objects in an image, and is therefore always a key consideration in composition. Factors include the relative sizes and positions of objects, and the extent of the space lying between them. A photographer must consider how these three-dimensional relationships should be captured in the two-dimensional image, otherwise the illusion of three dimensions will be reduced or lost. Every photograph has perspective but a photographer must use his or her skill to to transfer a sense of depth, distance, space and form to two-dimensional images. It is also possible, with suitable understanding of perspective, for a photographer to manipulate perspective compression and expansion and change the illusion of scale, depth and distance.
Various types of perspective can be identified. These might be summarized as:
The human eye and brain uses linear perspective to assess distances. We see the edges of a long straight road or the vertical walls of a building converge as they recede, and we see objects apparently becoming smaller as their distance from us increases. We interpret this information using our experience of life and are consequently able to make quite accurate estimates of the relevant distances and dimensions. Consequently it is important to understand how perspective changes with camera viewpoint, and how the choice of a particular focal length lens and camera-to-subject distance can be used to change both perspective and the apparent viewpoint. Note that changing the focal length of a lens from a fixed viewpoint does not change perspective. Using a wide-angle lens close to a subject produces an enhanced illusion of depth in a image. Using a lens having a long focal length at a large distance from a subject produces an illusion of compression of distances.
When we view a distant scene, for example a seascape, the water rises in our field of view as it recedes from the beach to the horizon. We are therefore able to estimate the distance to a boat by observing its position relative to its apparent distance above the beach and below the horizon. If the boat's waterline is almost as high up as the horizon we know that it is distant from the beach. If the boat 's waterline is lower down and closer to the beach, we know that the boat is nearer to our viewpoint. The boat's position on the vertical line from beach to horizon is known as its height perspective.
Experience of life teaches us that objects are normally of a certain size. We know that most adult human beings are about 1.5 - 1.9 metres tall and that a car may be 4.5 metres long and probably less than 2 metres in height. We also know the typical heights of trees and buildings. This knowledge allows us to gauge the relative distances of such objects. For instance, if a person appears to be taller than a building in front of which they are standing then we know that the building is much more distant than the person. This relative size information is very useful when observing a scene, and helps us to sort out where in the three-dimensional world various objects are positioned. This is known as size perspective.
Vanishing Point Perspective
Parallel lines, such as the two sides of a straight road, seem to converge as they recede into the distance. If a road is sufficiently long and straight the two sides eventually appear the merge or meet at the vanishing point. The vanishing points for horizontal or vertical parallel lines which are perpendicular to the lens axis are at infinity. All other lines meet at a specific vanishing point.
Three-dimensional objects illuminated by soft or diffused light reveal few clues about their form. When lit by direct sunlight they cast shadows and the parts that are directly and indirectly lit take on a somewhat different appearance. The length and depth of shadows, and their relative positions on the ground or surrounding objects, all provide depth and distance information. Such information provides us with an enhanced perception of form - the three-dimensional characteristics of the object
Objects that lie more or less on our line of sight are seen to overlap, those closer to our viewpoint overlapping and obscuring parts of more distant objects. This tells us that the obscured object is the more distant. Everyone has looked out to sea and wondered which of two converging ships is nearer and which is more distant. When we do not know the precise relative sizes of the two vessels, their relative distances may be very difficult to estimate. However when one finally overlaps the other it is easy to see not only which ship is closer but also their relative sizes and distances.
Rectilinear lenses, those that most people use all the time, produce rectilinear perspectives comparable with that of the human eye - ie straight lines are reproduced straight in an image. However some lenses are not rectilinear. Many fish-eye lenses produce a false perspective - all straight lines are curved. Panoramic lenses also produce a false perspective. Straight horizontal lines at the lens axis level are recorded as straight lines, but all other straight horizontal lines are reproduced as curved lines.
When photographing objects relatively close to the camera, air may be regarded as transparent but, when the subject is far distant, as in a scene of distant mountains or a picture taken from an aircraft, it becomes obvious that the atmosphere has a significant effect upon an image. This is due to the scattering of light as rays encounter particles of dust, water vapour, smoke and other forms of pollution. The scattering of light reveals itself in an image through the presence of a haze and a reduction in contrast and colour saturation, and an increase in brightness. The effect becomes more apparent as distance increases.
The brightness of objects may be increased by scattering because the scattering particles suspended in the air are themselves illuminated. This effect, when combined with the other effects of scattering, renders objects somewhat lighter in colour than they would be perceived closer up. Contrast is reduced by scattering because it is a consequence of the intensity of illumination and the reflective qualities of objects. The darker areas of a distant object may become less dark so contrast is reduced. This difference in contrast is easy to see and consequently helps our perception of relative distances. Colour saturation decreases with distance because pure colours are effectively diluted by scattered light which increases impure (grey) content. A reduction in colour saturation is also easy to detect and can be used to increase the perception of depth in an image. A reduction in image sharpness, brought about by scattering and lower contrast, may also be observed.