I.

Introduction

Photographic Techniques, various techniques of producing permanent images on sensitized surfaces by means of the photochemical action of light or other forms of radiant energy; or the more recent techniques of capturing images by electronic means.

In today's society photography plays important roles as an information medium, as a tool in science and technology, and as an art form, and it is also a popular hobby. It is essential at every level of business and industry, being used in advertising, documentation, photojournalism, and many other ways. Scientific research, ranging from the study of outer space to the study of the world of subatomic particles, relies heavily on photography as a tool. The history of photographic techniques shows that in the 19th century photography was the domain of a few professionals because it required large cameras and glass photographic plates; during the first decades of the 20th century, however, with the introduction of roll film and the box camera, it came within the reach of the public as a whole. Today the industry offers amateur and professional photographers a large variety of cameras and accessories. This development has been paralleled in the cinema by the changing techniques and technologies of cinematography.

Light is the essential ingredient in photography. Nearly all forms of photography are based on the light-sensitive properties of silver-halide crystals, chemical compounds of silver and halogens (bromine, chlorine, or iodine). When photographic film, which consists of an emulsion (a thin layer of gelatin) and a base of transparent cellulose acetate or polyester, is exposed to light, silver-halide crystals suspended in the emulsion undergo chemical changes to form what is known as a latent image on the film. When the film is processed in a chemical agent called a developer, particles of metallic silver form in areas that were exposed to light. Intense exposure causes many particles to form; weak exposure, a few. The image produced in this manner is called a negative because the tonal values of the subject photographed are reversed; that is, areas in the scene that were relatively dark appear light, and areas that were bright appear dark. The tonal values of the negative are reversed again in the photographic printing process or, when preparing colour transparencies (slides), in a second development process.

Photography, then, is based on chemical and physical principles. The sensitivity of silver halides to light is the primary chemical principle. The physical principles are governed by optics, the physics of light. The generic term “light” refers to the visible portion of a broad range of electromagnetic radiation, which includes radio waves, gamma rays, X-rays, infrared, and ultraviolet rays. The human eye is sensitive to only a narrow band of electromagnetic wavelengths, the visible spectrum. This spectrum comprises the full range of colour tones. To the eye, the longest visible wavelengths register as red, the shortest as blue.