When viewing the real world of reflected light we tend to overlook the obvious that the brightest areas are those reflecting the most energy. Early photographic processes were forced to deal with the fact that more light tended to produce more deformation of whatever sensitive material was chosen, lighter areas in the scene producing the greatest change, darker areas perhaps little or none.

Black and white photographic emulsions most typically contain a halide of silver. A "halide" is a chemical compound of a halogen (any of a group of five chemically related nonmetallic elements including fluorine, chlorine, bromine, iodine, and astatine) with a more electropositive element or group, in this case silver.

There are two steps in the making of a negative, as represented by the thin slice from the negative shown at the right.

1. EXPOSURE. The useful property of silver halide is that its state is altered when subjected to light, IN DIRECT PROPORTION TO THE AMOUNT OF LIGHT ENERGY ABSORBED. This change is not visible, and if film is examined before and after exposure, little change can be seen.
   
2. DEVELOPMENT. Silver halide WHICH HAS BEEN ALTERED BY CONTACT WITH LIGHT can be reduced to pure silver if placed in contact with specific chemicals referred to as developing agents.
   
   The activity of the developer and time of development will determine how much of the SENSITIZED halide will be converted. less above, more below.
   
   After development, some of the "altered" halide and all of the unaltered silver halide remains in the emulsion. It must be removed or the negative will darken and deteriorate over time. The removal of this undeveloped material is accomplished with fixing agents, usually sodium thiosulfate (hypo) or ammonium thiosulfate. The process is called "fixing". The trick is to "fix" just enough and not too much, as excessive contact with fixers can begin to remove some of the desirable silver material.

Discussion of the relationship between exposure and negative density was facilitated in 1890 by the research of Hurter and Driffield and their expression of the interaction in the form of a curve, to become known as the H & D Curve, or "characteristic curve".

Ideally, a photographic emulsion will react to light in a somewhat linear fashion, more exposure, greater density. Brighter parts of the scene provide more light and activate more silver halide in the emulsion.

This linearity fails for two reasons.

1. It takes a certain amount of light energy to initiate the activation of the photosensitive elements in the emulsion. Thus the density rises gradually at first in this area called the TOE, finally accelerating into the straight line portion of the curve.
2. With increasing exposure to light, more silver halide is converted, until it has no more sensitive material to activate. At that point, increasing the exposure does not increase the ultimate density of the developed negative. This "saturation" occurs gradually and produces what is known as a SHOULDER. It can be shown that even more exposure can produce a reversal leading to less density.

Each film emulsion reacts to light in a special way. Some react more quickly to low light than others creating a rather abrupt initial rise in density or "short toe". Others react more gradually to increases in light and have what is called a "long toe". Films with similar sensitivities and ranges can have quite different response curves requiring dissimilar exposure and development schemes. This discussion is further expanded in the development section.

Another important factor is the range of subject luminance that can be USEFULLY recorded. Long range films, like Tri-X can continue to build density over a long luminance range, whereas short range films, Kodachrome, Tech Pan, etc., saturate rather quickly and tend to "block" at either end.

Determining the precise film speed, coupled with precise exposure, is critical when the range of light in the scene is greater than the scale of the film (curve A), or approaches the scale of the film (curve B). If the colored bars represent the scene's range of light, it can be seen that ...

...a film with a scale of curve A may not record the scene's entire range of light. One must find an exposure which includes the best compromise and most nearly expresses the photographers vision. Films with a scale of curve B (covering the range of light in the scene, but not much more) require quite precise exposure in order to not lose data at either end. With long scale films, the issue is less demanding. A film with curve C would allow a variety of exposures (represented by the red, green, and blue bars). The slope of curve C is nearly the same for all three examples, which might represent using an ISO rating of 400, 200, and 100.