To begin with, visualize the world in two-dimensions. A photograph is merely a sheet of paper and has no depth, but must preserve the illusion of a third dimension if it is to be lifelike. Producing a really good two-dimensional representation of form and depth is one of greatest challenges in photography. Lighting is the key because visual experience allows us to relate tonal gradation in the flat image to the different ways in which surfaces respond to illumination in the real world. Areas of deeply contrasting light and shade emphasize depth, form and texture. However some subjects are best illuminated by diffused light that produces subtle effects. In black-and-white images shape, line, form, texture and tone are principal elements of composition and must be fully utilized to retain depth. In colour images brighter hues tend to advance, so brightness and saturation may become significant.
The edges of a photograph, the frame defined by the viewfinder, also have to be accommodated. Such boundaries do not exist in the environment, but all the elements included in an image have to lie comfortably within these strict limits. A couple of simple techniques help to develop the basic visual perception required. First we can learn to see and analyse shape, in a deliberate manner, independent of its real-world context. Partially closing the eyes helps to identify the principal shapes and elements in a scene. This diminishes the finer qualities of the light and reduces detail and colour saturation. Peering through a rectangular frame, perhaps formed by the thumb and first finger of two hands, can enhance the effect. Alternatively, just close down the lens aperture and look through the viewfinder.
The ability to see form in an analytical manner is a little more difficult to acquire, although it is basically only the three-dimensional equivalent of analysing shape. Key elements are how surfaces are illuminated and how they absorb or reflect light. Highlights, dark shadows and the infinite tonal gradations between the two extremes must be observed. Also, look at lines and curves, and consider whether they encourage the eye to explore the depth of the image.
The perception of colour is an integral part of seeing pictures. Colours alter balance, create the illusion of form, and suggest shapes and directions. Advancing, strident colours draw the eye, leap out of the picture and subordinate quieter hues. Consequently they are significant in the same way as the physical shapes of objects.
Finally, know your lenses and their limitations. An understanding of how they change the perception of a subject increases a photographer's control of image design. Telephoto lenses compress depth and enlarge the subject. Distant objects may look closer to the foreground than they really are, reducing uninteresting intervening spaces. Depth of field is restricted so differential focus can be used as an image design tool. Wide-angle lenses increase the impression of depth and scale, but diminish the size of the subject.