RAM

Temporary computer memory, used to store data and software while in use.

Rangefinder

A camera with a viewfinder separate from the lens. Not a SLR. Also, the usually built-in adjustable optical device for focusing a camera that automatically indicates the correct focus (as when two parts of a split image are brought together).

Raster

Images made of dots. Each individual one contains specific information as to its size, color and position within the image.

Raw

The RAW image format is the data from a digital camera as it comes directly off the CCD, with no in-camera processing performed.

Rear Focus

The focused area behind the subject.

Rear Focusing System

When focusing, only the rear lens group moves. This eliminates the changing of the physical length of the lens during focusing and allows for faster focusing. (Refer also to Internal Focusing.)

Rear Sync

In this mode, the flash fires when the second curtain starts to move across the frame, not the first one. When used in conjunction with slow shutter speeds a trail of light is left giving by blur effect the sensation of movement of a subject frozen by the flash.

Reciprocity Failure

Photosensitive materials require a linear sensitivity within a certain range. Beyond that their sensitivity requires that exposure times be increased in larger amounts than normal.

Recycling Time

The time it takes for a strobe or battery-pack to recharge so that it can power a flash burst. Shorter with fresh batteries and when shooting in power saving mode P.

Red Eye

The effect of red colored irises from subjects. It is caused by a combination of factors, low ambient light asking the irises to open more than regularly and small angle between the angles of both the lens axis and that of the flash light. The light bouncing from the back of the retinas brings back into the image the red of the blood vessels. Big eyes don't help either. Red eye can be reduced and even eliminated in several ways: by increasing the ambient light, increasing the angle between the lens and the flash beam with a bracket, having the subject look into a direction other than the camera, or into a bright light, or simply by using a flash with a red-eye system which pre-flashes to close down the irises of the subject.

Reflected Light Reading

Light meter reading made by pointing the meter towards the subject. It will vary depending on the subject as different materials reflect different amounts of light.

Reflector

A tool for redirecting light. Usually white or metallic, a cloth or any light-reflecting board.

Reflex Camera

A camera that uses a mirror to reflect light onto a ground glass for viewing and focusing.

RGB

Red, Green and Blue. The three colors to which the human visual system, digital cameras and many other devices are sensitive; the colors used in displays and input devices. They represent the additive color model, where 0% of each component yields black and 100% of each component yields white.

Relative Aperture

Diameter of the aperture divided by the focal length of the lens. Expressed numerically as an f-stop.

Release-Priority AF

In release-priority autofocus operation, the shutter can be released at anytime whether the subject is in focus or not. Used in fast-moving situations where you don't want to lose any of the action.

Repeating Flash

A feature available in some units to make multiple flash bursts during exposure. Useful for motion study in single frame multiple exposure. Best used in dark studios in Bulb setting.

Reproduction Ratio

The size of the subject in an image compared to its actual size. As a general rule, for subjects farther away than in macro photography, the focal length of the lens used, divided by the camera to subject distance yields the reproduction ratio. e.g. a 180mm lens focused at 1.8 meters (18000mm) will have a 1:10 reproduction ratio.

Resolution (Also known as Resolving Power)

The ability to reproduce small details in a photograph. Resolving power is used to measure lens performance using line pairs per millimeter (1/mm), and indicates how many black pairs of lines placed at equal intervals within 1mm can be resolved by a lens.

Retouching

To alter a finished print, digital image or piece of film in order to cover up undesirable marks or elements.

Reversal Materials

Photo-sensitive materials that when processed become positive images, i.e., slides and transparencies and certain print materials.

Ring Flash

A circular-shaped electronic flash unit that fits around a lens providing shadowless, uniform frontal lighting; especially useful in close-up photography.

Rising Front

The ability on a camera to raise the lens in relation to the film to control focus and distortion. Usually only on large format cameras.

Roll-Film

Non-sheet film. Film that comes in a roll and can be exposed in multiple "frames."

Roll-Film Adapter

An attachment for sheet film cameras that allows the use of roll film.