Digital Photography

A guide to common terms associated with digital photography.

Archiving: Preserving digital images in a way that is independent of where these records are kept. Image archives can consist of prints or copies on optical disk or hard disk drive.

Artefacts: Undesirable visual defects produced by digital imaging systems. They can be generated by either input or output devices and include noise, colour casts, distortions and lost information. All degrade image quality.

Aspect Ratio: The relationship between the horizontal and vertical dimensions of an image. The horizontal dimension is normally quoted first. A 35mm film frame has an aspect ratio of 3:2, while most digital camera images have a 4:3 aspect ratio. Many digital cameras and camcorders also offer a 'widescreen' format with a 16:9 aspect ratio.

Autofocus (AF): A camera control that focuses the lens on the subject. Two types of AF are in common use, infrared (IR) and contrast-based. The former fires a beam of infrared light at the subject and calculates its distance on the basis of the return reflection, while the latter evaluates distance on the basis of image contrast (close subjects have higher contrast than their background). Many cameras include servo-AF systems that can focus on a moving subject. This is also known as 'focus tracking'.

Audio Video Interleave (AVI): A video file format that commonly carries one video ('vids') stream and one audio ('auds') stream. Developed by Microsoft, it is the most common format for audio/video data on the PC.

Barrel Distortion: A type of image distortion that expands the central dimensions of the picture without affecting the periphery. It is most common in wide-angle lenses.

Batch Processing: A software tool that allows the same control or effect (or set of controls) to be applied to a number of digital files with only one command.

Bit: Short for 'binary digit', a bit is the smallest piece of information that can be handled by a computer and has a value of 0 or 1.

Bit Depth: The number of bits (binary digits) used to specify the brightness or colour range of each pixel in an image sensor.

Bitmap: A file format that records image data as individual pixels. Denoted by the .bmp extension.

Blooming: The halo effect that occurs at borders between dark and light image tones due to an overflow of electrical charge that is generated by excessive light exposure on part or all of the image sensor.

Buffer Memory: A special RAM storage area in a digital camera's memory system where image data is held while it awaits processing and transfer to the camera's memory card. A large buffer memory is required to support high-speed continuous shooting, especially at high image resolution.

Burst (Continuous) Shooting: A similar function to motor drive in a film SLR camera, which allows capture of a number of sequential shots in rapid succession. The number of frames that can be captured depends on the image resolution and the size of the buffer memory.

Calibration: The process of setting up equipment to give predictable output. This is an essential early step for good colour printing.
CCD (Charge-Coupled Device): A light-sensitive array of silicon cells that is commonly used for digital camera image sensors. It generates electrical current in proportion to light input and allows the simultaneous capture of many pixels with one brief exposure.

Chroma: The colour information associated with an image.

Cloning: The process of reproducing part or all of a digital image for transfer either to another image file or a different area within the same file.

CMYK: The basic colours used in four-colour printing. The letters 'C', 'M' and 'Y' stand for the subtractive primary colours; cyan magenta and yellow, while 'K' represents black, which is added to compensate for the lack of purity in C, M  and Y inks.

CMOS (Complementary Metal-Oxide Semiconductor): The alternative sensor array to CCD. CMOS sensors are cheaper to manufacture and use less power. Most digital SLR  cameras have CMOS image sensors, while the majority of digicams use CCD technology. 

Colour Filter Array: A mask, made of thin layers of dye that is applied over a digital camera sensor to enable it to record colour information. The individual colour patches filter out all but the chosen colour for that photosite and software interpolation is used to create a colour value for the resulting pixel based on surrounding pixel values.
The most common filter pattern is the Bayer array, which uses alternating rows of red/green and green/blue patches (GRGB).

Colour: The value produced by combining luminance (brightness) and chrominance (colour) signals.
Colour Management: Setting up a combination of software and hardware devices to produce accurate colour reproduction through all stages of a digital imaging system - from capture to output.

Colour Space: A geometrical system used to describe a range of colours. Adobe RGB (1998) and sRGB are the most commonly used colour spaces in digital imaging.

Compression: A mathematical processing system used to reduce the size of digital data files. Two types of compression are common in digital imaging: lossy (which sacrifices some data in order to obtain small files) and lossless (which involves little or no information loss).

Contrast: The difference between the lightest and darkest tones in an image. High-contrast images contain few steps between the lightest and darkest parts of the image, while low-contrast images contain many tonal gradations.

Crop: A manual or digital process that cuts away unwanted parts of an image.

Curves: Software tools that allow users to change colour and contrast values in images in a non-linear fashion.

Digital SLR (DSLR): A digital camera in which the subject is viewed through the same lens as the picture is taken with. A mirror is raised when the shutter button is pressed, allowing light to reach the image sensor. Most DSLR cameras use interchangeable lenses.

DPI (Dots Per Inch): The most commonly used unit of measurement for describing the resolution of digital image files for printing or scanning.

DPOF (Digital Print Order Format): Most digital cameras are compatible with the Digital Print Order Format (DPOF), a special type of metadata that lets users specify the photos they want printed by using the camera's menu system. The DPOF file is written to the camera's removable media, from which it can be read and executed by printing services and computer-based applications.

Dye Sublimation: Also known as 'thermal dye sublimation' or 'dye-sub', this colour printing technology forms images by a diffusion transfer process that requires transfer of gaseous dyes from a donor material to a receiving layer. Heat is required for the process. The output is near photographic in quality.

Dynamic Range: The measurable difference between the brightest highlight and darkest shadow area in an image that can be reproduced by an imaging system.

Effective Pixels: The number of pixels that are actually used to capture the image (as distinct from the total pixel count for the sensor). The remaining pixels (the difference between total and effective pixels) are used to provide a 'dark current reading' so the camera has a black reference point. The number of unused pixels is at the camera manufacturer's discretion, which is why effective pixel count is the only reliable guide to the camera's resolution potential.

Exif (Exchangeable Image File): A metadata format developed by the Japan Electronic Industry Development Association (JEIDA) that is closely associated with JPEG. Virtually any device or software application that stores or reads JPEG files is Exif compatible.

Exposure Value (EV): A number determined by the brightness of the subject and the sensitivity selected for the recording medium, it is larger for bright subjects and smaller for dark ones. When the amount of light doubles, the EV increases by 1, making the value equivalent to one stop of exposure.
File Format: The way in which digital information is saved by a software application. The most commonly used file formats for digital imaging are JPEG, TIFF and BMP (bitmap). Raw files are proprietary and, often, unique to each camera. Special software is required to decode raw files.

Filters: Software tools that are used to change the appearance of digital images locally or globally by adjusting the values or arrangement of certain pixels, either in the camera or in editing software.

Flash Memory: A special type of non-volatile RAM (random access memory) that stores information without requiring an electrical current. Most digital cameras store their image data on removable flash memory cards.

Gamma: The technical term used to describe image contrast, it refers to the slope of the line that represents image output values versus image input values. It is applicable to both film-based and digital images.

Gamut: The range of colours that an image contains or an output device can reproduce.

Histogram: A graphical display that shows the distribution of tones within an image. The horizontal co-ordinate represents the possible pixel values from black to white, while the vertical co-ordinate shows the number of pixels in the image at each value.

Horizontal Resolution: The number of pixels (for images) or lines (for video) a system can resolve along a horizontal axis.

Hue: The component of colour that relates to a specific wavelength or CIE co-ordinates.

Inkjet: A type of printer that applies microscopic ink droplets to paper to form images, graphics or text.

Interpolation: A mathematical re-sampling technique that is used to alter the size of an image file by creating or removing pixels on the basis of existing pixel values. Some quality is sacrificed as a result of the interpolation process, particularly if files are made larger.

JPEG: The image file format developed by the Joint Photographic Experts Group and denoted by the '.jpg' extension.

Lag: A term denoting delay after an action has been initiated. The most common lag times in digital imaging include shutter lag, autofocus (AF) lag and processing lag. Shutter lag describes the time taken for the camera to capture the shot after the shutter release has been pressed. AF lag defines the time it takes the camera to autofocus and processing lag describes the time images take to be processed and transferred to the memory card so the next image can be captured.

Landscape: A camera mode that sets the lens focus to near infinity, selects a small lens aperture and cancels the flash. For printing, the term defines a page orientation that is wider than it is high.

LCD (Liquid Crystal Display): LCD screens are used in digital cameras to preview and review shots. In some cameras they replace the optical viewfinder. Most also provide access to the camera controls via a set of menus, which are called up on screen and selected by pressing a button. Some cameras have separate LCD screens that display status information, such as frame counts, camera settings and battery power.

Lossless Compression: A compression technique that allows smaller files to be created without sacrificing the original data values.

Lossy Compression: A compression technique in which smaller files are produced by eliminating some of the original data. It produces smaller files than lossless compression and users can normally control how much data is sacrificed.

Metadata: Structured data, stored with digital image files, which explains, locates, describes or otherwise makes using the original primary data more effective or efficient. Two types of metadata are important for digital camera users: the Exif standard and the Digital Print Order Format (DPOF) standard.

Metamerism: A phenomenon in which two colours can appear the same under certain lighting conditions yet different under others. It is mainly associated with pigment-based inks for inkjet printers.

Motion JPEG: A type of AVI file format that handles images and sound as a single file, recording images in JPEG format. Motion JPEG can be played back by QuickTime 3.0 or later.

MPEG: A video compression system developed by the Motion Picture Experts Group, now in its fourth generation (MPEG4).

Native File Format: The default format in which a software application saves files.

Noise:  A random pattern of unwanted pixels that degrades the quality of image files.

Optical Resolution: The maximum physical (non-interpolated) resolution provided by a device. The term is commonly applied to scanners.

Optical Zoom: The maximum zoom range achievable with the camera's lens. Image quality is fully maintained.

Orientation: The direction in which a page is printed: landscape is printed horizontally, while portrait is printed vertically.

Photosite: The light-sensitive cell on a digital image sensor, it records one intensity and one colour value. Information from several photosites is required to create a pixel in the image.

Pincushion Distortion: An image aberration that compresses the centre of the field.

Pixel: Short for 'picture element', this term describes the basic component of a digital image. Individual pixels are generally square and carry one value for colour, luminance and intensity. Millions of pixels are required to produce a digital image that approaches photographic quality.

Posterisation: An editing process that reduces the number of tones in an image, creating a simplified - and often surreal - effect.

PPI (Pixels Per Inch): Often used interchangeably with DPI to describe the resolution of a digital image.

Raw Data: Digital information that has not been processed or formatted. Many high-end digital cameras provide a raw capture option, most using proprietary file formats that require special software to decode. Because they are un-processed, raw files are effectively 'digital negatives'.

Resolution: The ability to reproduce fine detail - or the amount of detail in the image.

RGB: A colour model based on the red, green and blue components in the output, it is typically used for images that will be displayed on monitors.

Saturation: The intensity of a hue. Pastels have low saturation, while bright colours are highly-saturated.

Sharpening: An image enhancement technique that gives more distinct edges to subject areas, lines and tones in a digital image. Sharpening can be applied in the camera or in editing software.

Sharpening Artefacts: Defects introduced by in-camera sharpening systems. These generally appear as white or black halos around high-contrast areas in the subject and can sometimes be minimised by turning off the auto sharpening function in the camera.

Thumbnail: A reduced-size, low-resolution version of a digital image, used mainly for sorting and retrieving image files.

TIFF (Tagged Image File Format): An image file format based on bitmapping that involves little or no data compression. Denoted by the .tif extension.

Tungsten Lighting: Light produced by either photofloods or domestic illumination, it has a Kelvin value of 3200 and is warmer than normal daylight.

USB (Universal Serial Bus): The most common way of connecting a peripheral device to a computer, USB offers 'hot' plug and play (you don't need to power-down the PC). The latest version USB 2.0 is significantly faster than the original USB 1.0 and USB 2.0 Hi-Speed is even faster.

VGA (Video Graphics Array): A video monitor with mid-level (640 x 480 pixels) resolution. Also applied to 640 x 480 pixel images.

WAVE: A standard Windows format for saving audio data, denoted by the .WAV file extension. WAVE  files can be either compressed or uncompressed and can be played back with Windows MediaPlayer or QuickTime 3.0 (or later) software.

White Balance: The control on a digital camera used to adjust the colour balance of the image to make shots look natural under a variety of different lighting conditions. Most cameras have pre-sets for tungsten and fluorescent room lighting plus daylight and open shade. A few also include a flash setting. Virtually all digital cameras have an auto white balance setting.

Zoom: A camera or software control that causes the image - or part of it - to appear larger (zooming in) or smaller (zooming out).