Rack and pinion focusing:- mechanical focusing system used on copying or monorail cameras. A pinion engages a rack on a slide. Focusing is achieved by turning a knob or wheel, which moves the lens or image panel.
Radiography:- technique of using X-rays, gamma rays and charged particles to form shadow images on photographic materials. Used in medical and industrial research because of its ability to penetrate opaque objects.
Rangefinder:- focusing system which measures the distance from camera to subject.
Rapid fixer:- fixing solution that uses ammonium thiocyanate or thiosulfate instead of hypo.
Rapid rectilinear:- lens system composed of two matching doublet lenses, symmetrically placed around the focal aperture. It was introduced by Dallmeyer and Sternheil and removed many of the aberrations present in more simple constructions.
Rayographs:- term coined by Man Ray and his friends for pictures made by placing directly on photographic paper (i.e. photograms).
Rear curtain sync:- when the flash fires an instant before the second or rear curtain of the focal plane shutter begins to move. When slow shutter speeds are used, this feature can create a blur effect from the ambient light, i.e., patterns following a moving subject with subject movement frozen at the end.
Rear focus:- refers to the focused area behind the picture's subject.
Rear focusing system:- system where only the rear lens group moves during focusing. It eliminates changes in the physical length of the lens during focusing. Rebate:- margin on photographic film surrounding the image area.
Reciprocity failure:- in photographic emulsions occurs when exposure times fall outside a films normal range. At these times an increase in exposure is required in addition to the assessed amount. This can be achieved either by increasing intensity or time.
Reciprocity law:- states that exposure = intensity x time, where intensity is equal to the amount of light and time is equal to the time that amount of light is allowed to act upon the photographic emulsion.
Reconstituted image:- photograph produced by translating light from the subject into electronic signals.
Recycling time:- time it takes a flash unit to recharge between firings.
Red eye:- effect encountered when light from a flash unit travels parallel to the lens axis during exposure.
Reducers:- solutions which remove silver from negatives and prints. They are used to diminish density and alter contrast on a photographic emulsion.
Reducing agent:- chemical in a developing solution which converts exposed silver halides to black metallic silver.
Reflected light:- light bounced off a subject, not falling on it.
Reflected light reading:- measurement by a light meter of the amount of reflected light being bounced of the subject. The light meter is pointed towards the subject.
Reflecting telescope:- telescope using a concave parabolic mirror to increase focal length and focus light at a point.
Reflections:- rays of light which strike a surface and bounce back again. Specular reflection occurs on even, polished surfaces; diffuse reflection occurs on uneven surfaces, when light scatters.
Reflector:- any substance from which light can be reflected. It also describes a white or gray card used to reflect from a main light source into shadow areas.
Reflex camera:- camera system which uses a mirror to reflect incoming image rays on to a ground glass screen, providing a system of viewing and focusing. See also SLR.
Reflex lens:- alternative term for mirror lens.
Refraction:- change in direction of light rays as they pass obliquely from one transparent medium to another of different density, e.g. air to glass.
Refractive index:- numerical value indicating the light bending power of a medium such as glass. The greater the bending power, the greater the refractive index.
Register:- exact alignment when overlaying separate images.
Register punch:- punched used to make alignment holes in film or paper for registering images.
Rehalogenization:- process by which black metallic silver is converted back to silver halides. It is used in bleaching for toners and intensification.
Relative aperture:- measurable diameter of the diaphragm divided by the focal length of the lens in use and expressed in terms of "f" numbers, marked on the lens barrel.
Replenishment:- addition of chemicals to a processing solution to maintain its characteristics, e.g. developers are replenished with reducing agents as the old ones are exhausted through use.
Resin coated paper (RC):- printing paper with a water repellent base. RC Paper can be processed faster, require less washing, and dry more quickly than fiber based papers.
Resist:- protective but removable layer applied to a surface in the form of a pattern or image. Used to prevent chemicals solutions reaching covered areas.
Resolving power:- ability of the eye, lens or photographic emulsion to determine fine detail. In photography, the quality of the final image is a result of the resolving power of both the lens and the sensitive emulsion. Resolution is expressed in terms of lines per millimeter which are distinctly recorded or visually separable in the final image.
Restrainer:- chemical constituent of developing solutions which helps prevent reducing agents from affecting unexposed halides and converting them to black metallic silver.
Reticulation:- regular crazed pattern created on the emulsion surface of negatives which is caused by extreme changes of temperature or acidity/alkalinity during processing.
Retrofocus:- type of lens design with a negative lens element positioned in front of the diaphragm and a positive lens element positioned at the rear of the diaphragm. This makes the distance from the rear of the lens to the focal plane longer than the lens focal length. Retrofocus design has been adopted in wide angle lenses so the rear of the lens does not impede the movement of an SLR camera's reflex mirror.
Retouching:- after treatment carried out on a negative or print, in the form of local chemical reduction, local dye or pencil additions or air-brushing. The purpose is to remove blemishes on the negative or print.
Reversal materials:- materials specifically designed to be processed to a positive after one camera exposure.
Ring flash:- ring shaped electronic flash unit attached to the front of a lens. Used to give even frontal lighting in closeup situations.
Rinse:- brief clean water wash between steps of a processing cycle to reduce carry-over of one solution into another.
Rising front:- camera movement enabling the front lens panel to be raised or lowered from its central position on most view cameras.
Rods:- receptors forming part of the retina at the back of the eye sensitive only to variations in brightness, not color.
Roll film:- refers to 120, 220 and 620 film formats.
Roll film adaptor:- specially designed attachment for cameras designed for cut film, enabling roll film to be used.