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- RAM
- Random Access Memory. Short-term computer memory, it's where the computer's CPU stores software programs and data that are currently being used.
- Rangefinder
- A type of camera that features a focusing method involving the display in the viewfinder of two separate images of the subject. Sharp focus is achieved when the focusing mechanism is turned and the images coincide.
- Raster
- Refers to images made up of individual dots, each having a defined value that identifies its size, color and place within the image.
- RAW
- An Image format in which the data comes directly from the camera's CCD, unmodified, with no in-camera processing performed.
- Rear-Curtain Sync
- Sometimes called second curtain sync, it's a technique in which the flash fires an instant before the second (rear) curtain of the camera's focal plane shutter begins to move.
- Rear Focus
- Refers to the focused area behind the picture's subject.
- Rear Focusing System
- A system in which only the rear lens group moves during focusing, thus eliminating changes in the physical length of the lens during focusing and enabling faster focusing.
- Reciprocity Failure
- A phenomenon that occurs when film is exposed under conditions that are not within its practical brightness range.
- Recycling Time
- The time necessary for an electronic flash to recharge and be ready for the next picture, it often depends on the type and condition of the battery and the guide number of the flash.
- Red-Eye
- The phenomenon in which the center portions of a subject's eyes appear as red spots in flash pictures.
- Reflected-Light Exposure Meter
- Measures light reflected from the subject. Reflected-light readings are affected by the amount of light in the scene and the subject’s reflective properties.
- Reflector
- An object or device—it can be a piece of white paper or cloth or a foil-wrapped piece of cardboard—that directs light toward or away from a subject. Often used to fill in shadows; also often used for outdoor portraits.
- Reflex Lens
- A reflex lens employs a combination of mirrors and lens elements to "fold" the light path, permitting a reduction in overall lens length and weight compared with conventional lenses of equivalent focal lengths.
- Reflex Mirror
- The mirror that reflects the light passing through the lens upward to the prism for viewing.
- Relative Illumination
- Illumination around the periphery of the image field. Used as a comparison of the brightness between the center and the periphery.
- Release-Priority AF
- An autofocus mode in which the shutter can be released anytime, even if the subject is not in focus. Recommended in fast-moving situations when you do not want to miss any of the action.
- Removable Memory
- A solid-state removable memory card for a digital camera that comes in a variety of size and capacity configurations. The larger the card's capacity, the more digital pictures it can store.
- Repeating Flash
- A feature available in some flash units that fires the flash many times during a brief exposure.
- Reproduction Ratio
- Specifically the size of the image recorded on film divided by the actual size of the subject.
- Resolving Power (Resolution)
- The ability of a lens or photographic material to reproduce small details. One of the measurements of lens performance.
- Retrofocus
- A type of lens design with a negative (diverging) lens element positioned in front of the diaphragm and a positive (converging) lens element positioned at the rear of the diaphragm.
- Reversal Film
- Also called positive film or slide film. When exposed and processed, images appear similar to the subject in tone and color.
- Reverse Adapter
- See Macro Adapter Ring.
- RGB
- Referring to red, green and blue, the primary colors used on a computer monitor to simulate natural color.
- Right-Angle Viewing Attachment
- An accessory for the viewfinder eyepiece that enables viewing from a right angle. Useful for copystand work and when the camera is inconveniently positioned low, near the ground, or at an angle which makes normal viewing impossible.
- Rule of Thirds
- More a composition guideline than a rule, here's how it works: Imagine the lines of a tic-tac-toe game superimposed over the scene you're viewing in the finder.