Rangefinder1.) A device on a camera that measures the distance from camera to subject and shows when the subject is in focus.  2.) A camera equipped with a rangefinder focusing device.  Abbreviated RF.

RC paper:  See resin-coated paper.

Reciprocity law The theoretical relationship between length of exposure and intensity of light, stating that an increase in one will be balanced by a decrease in the other.  For example, doubling the light intensity should be balanced exactly by halving the exposure time.  In fact, the saw does not hold true for very long or very short exposures.  This reciprocity failure or reciprocity effect causes underexposure unless the exposure is increased.  It also causes color shifts in color materials.

Reducing agent:  The active ingredient in a developer.  It changes exposed silver halide crystals into dark metallic silver.  Also called the developing agent.

Reduction 1.) A print that is smaller than the size of the negative. 2.) The part of development in which exposed silver halide crystals forming an invisible latent image are converted to visible metallic silver.  3.) A process that decreases the amount of dark silver in a developed image.  Negatives are usually reduced to decrease density.  Prints are reduced locally (only in certain parts) to brighten highlights.  Opposite:  intensification.

Reel:  A metal or plastic reel with spiral grooves into which roll film is loaded for development.

Reflected-light meter:  An exposure meter that measures the amount of light reflected or emitted by a subject.  Sometimes called a luminance meter.

Reflector:  1.) A reflective surface, such as a piece of white cardboard, that can be positioned to redirect length, especially into shadow areas.  Also called a flat.  2.) A reflective surface, often bowl-shaped, that is placed behind a lamp to direct more light from the lamp toward the subject.

Reflex camera A camera with a built-in mirror that reflects the scene being photographed onto a ground-glass viewing screen.  See also single-lens reflex; twin-lens reflex.

Replenisher:  A substance added to some types of developers after use to replace exhausted chemicals so that the developers  can be used again.

Resin-coated paper Printing paper with a water-resistant coating that absorbs less moisture than un-coated paper, consequently reducing some processing times.  Abbreviated RC.

Reticulation:  A crinkling of the gelatin emulsion on film that can be caused by extreme temperature changes during processing.

Reversal:  A process for making a positive image directly from film exposed in the camera; also for making a negative image directly from a negative or a positive image from a positive transparency.

Reversal film:  Film that produces a positive image (a transparency) on exposure and development.

RF:  See rangefinder.

Roll film:  Film that comes in a roll, protected from light by a length of paper wound around the film.  Loosely applies to any film packaged in a roll rather than in flat sheets.