The resultant contrast as recorded on the film is described as the 'density difference’ between the lowest density (darkest shadow) and the highest density (brightest highlight) on a negative.

Subject The maximum and minimum reflectivity of the subject. The photographic image is a recording of the amount of light reflected from an object. If one part of an object has 90 per cent reflectance and another part 5 per cent, the density difference recorded on the film will be greater than if one part reflects 70 per cent and another part 20 per cent.
Lighting Strong direct light will give harsher, deeper shadows and a greater density difference than soft, diffused light.
Optics The materials and design of optics can reduce or increase the contrast as the image is refracted. Focus is also a consideration at this stage.
Flare Non image forming light. Reflections on the surface or within the optics or camera/printer interfere with the image. This can be equated with static on the radio.
Inherent Contrast The film manufacturers design the films to have certain characteristics that make them particularly suitable for certain tasks: e.g. duplicate negative. One of the most important design characteristics is the inherent or 'built in’ contrast. If a film with a high inherent contrast is drastically underdeveloped, the contrast would still be higher than a low inherent contrast film, even one that had been overdeveloped. The contrast can be varied by development, but only within a limited range: e.g. for dupe neg changes in development may be able to vary the gamma between 0.4 and 0.9.
Development The time, temperature and formulation/condition of the developer.

Table 13.4: Factors affecting contrast

Measurement of developed contrast

To measure the developed contrast (gamma method) of a processed film using a characteristic curve, the tangent of the slope of the straight line portion is calculated.

Fig 3.9 Simple method of calculating gamma

The simplest method of doing this is to extrapolate the straight line portion to the base line or x axis and determine the tangent of the angle formed.

The straight gamma method does not take into account any of the toe characteristics of the emulsion, which may vary from emulsion to emulsion. As it is a 'best fit’ method through plotted data points personal interpretation may alter the result.

The method recommended by Eastman Kodak uses calculated density points to construct a straight line from the curve. A different equation is used for each stock type to allow for toe and other characteristics.