Digital Imaging Glossary

A guide to common terms associated with digital photography.

AE and AF Locks: Button controls on a camera that allow photographers to lock the exposure on a different part of the subject from the point of focus – or vice versa. In most cameras, pressing the shutter button halfway down locks both the AE (auto exposure) and AF (autofocus) settings.

Ambient Light: The light that exists in a specific situation without augmentation with flash or studio lights. Also known as ‘available light’.

Aperture: The opening in the iris diaphragm of a lens that allows light to pass through the lens to the image sensor.

Aperture-priority: Usually denoted by the A (or Av) setting on a camera’s mode dial, this shooting mode allows the photographer to set a specific aperture value while the camera will adjust the shutter speed automatically to ensure a correct exposure. The main purpose of this shooting mode is to control depth-of-field.

APS-C: A term originally developed for the ‘Advanced Photo System’ film format but now used to define a digital imaging sensor size that measures between 21.5 x 14.4 mm and 23.7 x 15.7 mm in area.

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, as do most DSLR cameras. Images from Four Thirds System DSLRs and compact digicams 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, active infrared (IR) and passive 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’.

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.

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. JPEG images are always recorded with 8-bit depth, which can record 256 (28) levels of red, green and blue. Cameras that support raw file capture offer higher bit depths, usually ranging from 12 to16 bits.

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.

Bracketing: An exposure technique that involves taking a series of shots with slightly different camera settings from those determined by the camera’s automatic measurements. Most cameras provide bracketing controls for exposure and some also provide white balance and focus bracketing.

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 function that allows a camera to record 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.

Centre-weighted Average Metering: An exposure metering pattern that integrates readings from all over the field of view, placing more emphasis on the centre of the field.

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.

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.

CompactFlash (CF): A type of camera memory card that is commonly used in DSLR cameras. CF cards measure 43 x 36 mm in area and most are 3.3 mm thick. They come in capacities up to 64GB.

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.

Depth-of-Field: The area in a scene that appears acceptable sharp in a photographic image. Depth-of-field is controlled by the lens aperture. It is greatest with distant subjects, wide-angle lenses and small lens apertures and least with close subjects, telephoto lenses and large lens apertures. Some DSLR cameras have a dept-of-field preview button that temporarily closes the iris diaphragm so photographers can see the depth-of-field they will get in a shot.

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: A printing technology that uses dye-transfer to produce coloured prints. Most printers are restricted to snapshot-sized output.

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.

Exposure: A term used to describe the combination of lens aperture and shutter speed that delivers a pre-determined amount of light to the image sensor. All cameras include exposure meters, which measure the tones in the subject according to a selected pattern.

Exposure Compensation: A camera setting that allows photographers to over-ride the settings used by the camera to reduce or increase the overall exposure value. The control is indicated by a +/- icon, either on a button or in a menu.

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.

Fast lenses: Lenses with large maximum apertures – typically f/1.8 to f/2.8. Because they require more – and better quality – glass, fast lenses command premium prices.

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: Add-on accessories that are used to change the appearance of digital images as they are recorded.

Flare: An imaging problem cause by light scatter within a lens. It is commonly seen in photographs of backlit subjects and may show up as bright or coloured spots on the subject or an overall reduction in contrast. Flare is reduced by coating the lens elements and using a lens hood to prevent stray light from entering the lens.

Flash Synchronisation: A control that ensures the flash is fired when the camera’s shutter is open. Modern cameras often offer several flash synch settings; typically slow synch, which engages a slow shutter speed to allow background details to be recorded, first-curtain synch, which fires the flash just after the shutter opnes and rear-curtain synch, which fires the flash just before the shutter closes.

Four Thirds System: A digital SLR system developed by Olympus, Kodak and Panasonic that centres on an image sensor size of 18.0 x 13.5 mm and uses a special, non-propriteary lens mount.

‘Full Frame’ Sensor: An image sensor measuring 36 x 24 mm in area – which has the same surface area as a 35mm film frame.

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.

Graduated Filters: Graduated filters are filters with variable light transmission. Typically half of the filter area is darker or a different colour while the rest is clear. They are used to darken overly-bright skies to ensure scenic shots have a natural balance between sky and land tones.

Highlight & Shadow Alerts: Playback settings that allow images to be displayed with blinking highlights and/or shadows so photographers can see whether shots have been correctly exposed.

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.

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

Image Noise: A reduction in image quality that is usually seen as graininess and/or tiny white and coloured dots. It is caused by random fluctuations in the digital signal and associated with high ISO sensitivity settings and long exposure times.

Image Processor: The computer chip in a digital camera that converts the analogue signal from the image sensor into a digital picture.

Image Sensor: The element in a digital camera that records the digital image. Two types are popular: Charge-Coupled Devices (CCD) and Complementary Metal-Oxide Semiconductor (CMOS). CCDs are universally used in compact digicams while CMOS sensors are more common in DSLR cameras.

Image Stabilisation: A system for reducing the effect of camera shake. Two technologies are in common use, one compensating by moving elements in the lens and the by moving the sensor. The former is more effective.

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

Internal Focusing (IF): A system for focusing a lens by moving internal elements. It allows the lens to remain the same length and its barrel does not rotate while focusing occurs, allowing angle-critical accessories like polarisers and graduated filters to be used.

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.

Iris diaphragm: The structure controlling the aperture in a camera’s lens. The ratio between the diameter of this aperture and the focal length of the lens is given in an f/number.

ISO: The International Standards Organisation’s system for defining sensor sensitivity. The system works by doubling. ISO 200 is double the sensitivity of ISO 100 and half the sensitivity of ISO 400.

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

Kelvin (K): Colour ‘temperature’ is measured on a Kelvin scale, in which colours are denoted by the temperatures at which a heated black-body radiator matches the colour of the light source. This system allows precise colour values to be specified.

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.

Lens: The optical device consisting of several glass elements which transmits and refracts light to produce the image that is recorded by a camera’s sensor. DSLR cameras use interchangeable lenses.

Live View: A function on all compact digicams and many recent DSLR cameras that allows the LCD monitor to be used for composing photographs.

Manual: The Manual (M) setting on a camera’s mode dial gives the photographer full control over the lens aperture and shutter speed settings.

Megapixels: A term used to describe one million pixels. It is used to define the number of pixels in a digital image and also (erroneously) to express the number of photosites in the image sensors in digital cameras.

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.

Monochrome: Refers to an image that is all one colour, typically black and white (with intermediate grey tones) or sepia. When images are recorded in monochrome, colour information is discarded.

Multi-pattern Metering: An exposure metering pattern that divides the subject area into five or more segments and individually evaluates the light levels within each segment. Exposure settings are determined by balancing the readings from each segment.

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

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.

Playback Zoom: A camera function that enables photographers to enlarge part of an image during playback to check focusing and exposure levels.

Polarisers or Polarising Filters: Polarising filters are used to reduce the effects of scattered light and, thereby, brighten colours and allow blue skies to be reproduced with a natural appearance. They can also reduce the effect of specular reflections off water or other shiny surfaces. Two types are available: linear and circular. Circular polarisers are recommended for photography.

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

Prime Lens: A lens that covers only one focal length.

Pro-sumer: Between professional and consumer (usually referring to camera equipment).

Raw Data: Digital information that has not been processed or formatted.

Raw Files: Raw files contain the image data as it is captured by the camera’s sensor with only minimal processing applied. 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.

Secure Digital (SD): A type of camera memory card that is used in some DSLR cameras. SD cards measure 32 x 24 x 2.1 mm and come in capacities up to 32GB. A high-capacity version, SDHC (Secure Digital High Capacity) was launched in 2006 and offers faster data transfer speeds.

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.

Shutter Priority: Usually denoted by the S (or Tv – for time value) setting on a camera’s mode dial, this shooting mode allows the photographer to set a specific shutter speed while the camera will adjust the lens aperture automatically to ensure a correct exposure. The main purpose of this shooting mode is to control the ways in which motion is depicted. Fast shutter speeds are used to ‘freeze’ movement, while slow shutter speeds record movement as a blur.

Spot Metering: An exposure metering pattern that takes a single reading from a small section of the field of view.

Telephoto: A term used for lenses with focal lengths greater than 70mm.

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 640 x 480 pixel resolution. Also applied to 640 x 480 pixel images.

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).

Zoom lens: A lens that covers a range of focal lengths. Zoom lenses are usually several stops slower than prime lenses, particularly at longer focal lengths.

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