- Impressionism - art movement in which painters broke away from the techniques of continuous brushstrokes and clearly expressed detail. They were largely concerned with the effects of light and color.
- Infectious development - development action which occurs in processing "lith" materials. The oxidation of hydroquinone produces new and highly active reducing agents, semiquinones, in the presence of a low quantity of sodium sulfite. This results in a very high contrast image.
- Intensification - chemical method of increasing the density of the photographic image. It is only suitable for treating negative materials and works better on negatives that have been underdeveloped rather than underexposed.
- Intermittency effect - states that, a number of short, separate exposures will not produce the same photographic result when combined as a single exposure of equivalent total duration.
- Intersection of thirds - compositional technique whereby the image area is divided horizontally and vertically into equal thirds by means of four imaginary lines. The main subject is considered strongly placed it it is positioned at the intersection of any two of these lines.
- Inverse square law - states that, when the light source is a point, illumination on a surface is inversely proportional to the square of the distance of the light source.
- Iodine - chemical used in reducers and bleachers.
- Isoionic Point - The pH where the concentration of the dipolar ion is at a maximum
- Ivorytype - obsolete printing process designed to give the impression of a painting on ivory. A hand colored print was impregnated with wax and squeegeed face down on hot glass. The paper base was then back by ivory tinted paper.