to be further elaborated
Human vision is far superior to any camera in terms of the range of tones it can encompass within a single field of vision.
Early photographic emulsions were considerably more sensitive to blue light than to other colours on the spectrum of visible light. This meant that landscape photographs, particularly those made on clear days, had completely blown-out skies as the negatives were much denser in the skies than the foreground, resulting in loss of detail in the (positive) print.
Edward Muybridge made a library of clouds and skies that would be layered with a negative where the sky detail was absent in order to make photographs that were nearer to human perception.
The Zone System by Ansel Adams and Fred Archer (1889-1963) is a way to visualise how the tones visible in a scene can most effectively be rendered onto the photographic negative.
Adams (1981, 60) described the zone scale and its relationship to typical scene elements:
|I||Near black, with slight tonality but no texture|
|II||Textured black; the darkest part of the image in which slight detail is recorded|
|III||Average dark materials and low values showing adequate texture|
|IV||Average dark foliage, dark stone, or landscape shadows|
|V||Middle gray: clear north sky; dark skin, average weathered wood|
|VI||Average Caucasian skin; light stone; shadows on snow in sunlit landscapes|
|VII||Very light skin; shadows in snow with acute side lighting|
|VIII||Lightest tone with texture: textured snow|
|IX||Slight tone without texture; glaring snow|
|X||Pure white: light sources and specular reflections|
Adams (1981, 52) distinguished among three different exposure scales for the negative:
- The full range from black to white, represented by Zone 0 through Zone X.
- The dynamic range comprising Zone I through Zone IX, which Adams considered to represent the darkest and lightest “useful” negative densities.
- The textural range comprising Zone II through Zone VIII. This range of zones conveys a sense of texture and the recognition of substance.
Adams and Archer sought to refine and better manage some if the many variables that affected exposure, such as developer formulae and development times, so that the photographer could more strictly control the contrast and range of tones rendered.
In reality both film and digital sensors can render many more ‘zones’ than just eleven. But reminds us that when you point a light meter at an object it reads it as mid grey (zone 5). Therefore the photographer has to decide where in the scene they wish Zone V to be in order to control exposure properly.
In colour photography this also needs to be adjusted to allow for the fact that different colours correspond to different tones – yellows are better slightly over-exposed while reds and blues under-exposed.