The Zone System -Concepts
The Density - EI Relationship
Before dealing with the actual procedures of the zone system, it is desirable to introduce a few concepts.
Lets begin by looking at the way in which an exposure meter operates. A 35mm camera with a built in meter takes account of a wide variety of tonal values within the scene and averages then sets the exposure to this average. This average is actually all the tonal values added together to make a mid grey tone. In the zone system this is called
Zone V, or mid gray and has a reflectance value of 18%.
However this system of taking a meter reading assumes that the tonal values of the scene will average out to give a correct exposure. If we are photographing a scene that is very light or very dark this average will sway one way or the other and give us an inaccurate exposure.
If we use a spot meter, which measures the light coming from a small section of the scene, in fact about one degree in diameter, we are able to pin point the exposure to a particular tone within the scene rather than average them all out. If the reading is taken from an area that is mid gray or Zone V, it could be argued
that this is the simplest use of the zone system.
Normally the ISO is set on the meter, which is the official speed rating of the film
You set some value of the exposure index (EI) into the meter, then take a reading from some particular portion of the scene. Then The exposure meter then offers you a large number of aperture/shutter speed combinations, all of which would give the same exposure on the film at the point on the film image corresponding to the point on the scene which you metered. On your meter, the EI is probably called "ISO". However, in this method, we will be setting all sorts of values for this speed parameter, both higher and lower than the official ISO, so the term EI will be used to avoid confusion.
If we were to expose a negative using one of these aperture/shutter combinations, then developed the negative in some specific developer, at some specific dilution, and at a specific time, temperature and agitation method, there will result some specific density on the film at the point being considered.
Zone 0 Total black
Zone I Black without any texture
Zone II Black with slight suggestion of tonality
Zone III Darkest areas that still retail some visible detail
Zone IV Average shadows in landscapes or portraits
Zone V Middle Gray - 18% gray card
Zone VI Average Caucasian skin - Shadows on snow in sunlit snowscapes
Zone VII Lightest areas in any scene that still retain some visible detail
Zone VIII White areas with slightly visible textures - Highlights on Caucasian skin
Zone IX Glaring white surfaces - Highlights without any texture
Zone X A light source (records only as the maximum white value of paper surface)
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