Optics presents distant objects as being comparatively smaller than identical ones nearby, and yet, our brain is able to understand them as being small and yet full-scale simultaneously. Multiple identical points arranged into two lines equidistant from each other; for example, milestones on either side of a straight road, will appear to get closer to each other (converge) as we travel further away from them, even though we know they remain the same distance apart. This is the perceptual phenomenon of "scale constancy".
We should have a conscious knowledge of the perspectival clues our eyes recognise when comprehending depth, as a necessary first step to being able to control the feeling of depth in an image.
- Linear perspective
Two or more parallel lines converge or imply convergence at a distant vanishing point. This applies vertically as well as horizontally, but the latter is easier for the eye to accept.
- Diminishing perspective
A special instance of linear perspective, where objects of known scale have aspects in inverse proportion to their distance from the viewer e.g. a single row of trees receding into the distance.
- Aerial perspective
Contrast and colour tone diminish as distance increases due to diffusion by atmospheric haze.
- Colour perspective
Air absorbs warmer-hued light (longer wavelengths) than colder-hues; we perceive warmer colours as advancing and cooler colours as receding.
- Tonal perspective
As a result of our reality-exposure to atmospheric diffusion, we perceive higher contrasts as being closer to us and lower contrasts as farther away.
Diffusion of light due to atmospheric haze makes the outlines of distant objects less sharp.