NSL Photography's™ Glossary of Photographic Terms - R

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Rangefinder
Instrument for measuring distances from a given point, usually based on slightly separated views of the scene provided by mirrors or prisms.

Raw Image File
A raw image file contains minimally processed data from the image sensor of a digital camera. Raw file images are not yet processed and therefore are not ready to be used with a bitmap graphics editor or printed. Normally, the image is processed by a raw converter in a wide-gamut internal colorspace where precise adjustments can be made before conversion to a film format such as TIFF, PSD or JPEG (JPG) for storage, printing, web display or further manipulation, which often encodes the image in a device-dependent colorspace.

Raw image files are sometimes called digital negatives, as they fulfill the same role as negatives in film photography, that is, the negative is not directly usable as an image, but has all of the information needed to create an image.

Like a photographic negative, a raw digital image may have a wider dynamic range or color gamut than the eventual final image format, and is usually the one "closest" to the real picture as it preserves the most of its details. The raw image format's purpose is to faithfully record both 100% of exactly what the sensor "saw" or "sensed" (the data), and the conditions surrounding the recording of the image (the metadata).

Rear-Curtain Sync
Flash fires an instant before the second (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., a flowing- light patterns following a moving subject with subject movement frozen at the end of the light flow.

Reciprocity Failure
This occurs for photographic emulsions (film) when exposure times are beyond the film's normal range. As exposure times increase, more light, and/or more intensive light is required in addition to the expected amount, to properly expose the film to achieve the desired result.

Rectilinear lens
This is a photographic lens which produces images where the straight lines of the subject, such as building walls, appear straight, not curved. These lenses have little barrel or pincushion distortion. The majority of still camera lenses produce nearly rectilinear images.

Refractive Index
A technical term used to describe the effect of a lens in causing light rays to bend.

Reflector
Any device used to reflect light onto a subject to improve balance of exposure (contrast). Another way is to use fill in flash.

Relative Aperture
Numerical expression of effective aperture, also known as f-number. It's obtained by dividing focal length by diameter of effective aperture.

Remote Shutter Release
A mechanical or electronic device which attaches to a camera for the purpose of activating the shutter of the camera without touching the camera, to eliminate shaking the camera from the physical action of pressing the shutter release button. Remote shutter releases may vary from a simple physical cable which presses down on the shutter release button, to an electronic remote shutter release which can both delay the shutter release for a set period of time, and keep the shutter open for a set period of time.

Resolution
The ability of a lens to discern small detail. In photography, the image resolution in the final photograph depends on the resolving power of the film or the sensor, and of that of the lens. The two are not related, but the effective resolution is a function of both.

Retouching
Altering a print, image or negative after development by use of dyes or pencils software, to alter tones of highlights, shadows, and other details, or to remove blemishes.

RGB
Red, green & blue, where black is simulated color. CMYK is the four primary colors.

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