Aspect Ratio: The relationship between the horizontal and vertical dimensions of an image. A 35mm film frame has an aspect ratio of 3:2, which is the same as a standard 15 x 10 cm print but doesn’t match the aspect ratio of A4 printing paper. Most digital cameras produce images with a 4:3 aspect ratio, although many also offer the 16:9 aspect ratio used by widescreen TV sets and some add 1:1, which produces square pictures. When the image aspect ratio does not match the aspect ratio of the printing paper, the image or print may need to be cropped or you may elect to print the picture smaller than the paper, with white borders around it.
Bit Depth: The number of bits used to specify the brightness (for B&W) or colour range of each pixel in an image. Images containing eight bits of information can record 256 tonal gradations. When we speak of 8-bit colour images, we really mean eight bits of blue, green and red data (24 bits in all), which is equivalent to 16 million distinct hues and tones. Professional cameras and software and most scanners can capture images at higher bit depths (12-bit or 16-bit) but the advantages of this can only be realised in software that can handle the additional data. The resulting image files are roughly twice the size of 8-bit files and provide a better platform for enlargement to A3 size or bigger than 8-bit images.
Bitmap: A file format that records image data as individual pixels. Denoted by the *.bmp extension.
Calibration: The process of setting up equipment to give predictable output. The first step in producing good colour prints is to calibrate the monitor you use for editing the images.
Chroma: The colour information associated with an image.
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.
Collage: A collection of images printed on a single sheet of paper, usually arranged informally with many pictures overlapping.
Colour Management: A process by which the various devices in an imaging chain (camera, monitor, editing software, scanner, printer) communicate colour information to provide consistent colour reproduction from capture to output.
Colour Space: A geometrical system used to describe a range of colours. The universal colour space used by all imaging devices is sRGB. Adobe RGB (1998) is an alternative colour space provided in higher-featured digital cameras and supported by more sophisticated editors and printers. It is popular with landscape photographers because it covers a wider colour gamut.
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).
Direct Printing: A system that enables a printer to be used as a stand-alone device without requiring a computer to be connected. Popular systems include the provision of memory card slots in the printer itself and the PictBridge system found in most digital cameras, which gives the printer access to image files stored in the cameraø¢â‚¬â„¢s memory (or on the card in the camera).
DPI (Dots Per Inch): The most commonly used unit of measurement for describing the resolution of digital image files for printing or scanning.
Driver: Dedicated software that allows a computer to interface with and control another device, such as a printer or scanner.
Dynamic Range: The measurable difference between the brightest highlight and darkest shadow area in an image that can be reproduced by an imaging system.
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 a special file format that records the raw data captured by the camera’s image sensor without applying any further processing.
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.
ICC (International Colour Consortium): An industry consortium which has defined an open standard for a Colour Matching Module (CMM) at the operating system level, and colour profiles for the devices and for the colour spaces the user edits in. It also defines the device-linking profiles that underpin colour management systems.
Interpolation: A mathematical re-sampling technique that is used to increase the size of an image file by creating more 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 through interpolation.
JPEG: The image file format developed by the Joint Photographic Experts Group and denoted by the .jpg extension. JPEG uses compression algorithms to reduce file sizes, sacrificing some image data to obtain small files. An updated version, JPEG 2000, was launched in 2000 but has been largely ignored by camera manufacturers, although some software supports it.
Lightfastness Rating: The length of time an image can retain its colours and dynamic range, usually defined in relationship to certain display characteristics. Ratings will differ depending on whether the image is protected from exposure to light and atmospheric pollutants.
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.
Optical Resolution: The maximum physical (non-interpolated) resolution provided by a device. The term is commonly applied to lenses and scanners.
Orientation: The direction in which a page is printed or an image is viewed: landscape is printed horizontally, while portrait is printed vertically. Most printers’ paper feed systems require paper to be inserted in portrait orientation (with the shorter side of the paper first).
Pixel: Short for picture element, this term describes the basic component of a digital image. Individual pixels are generally square and carry single values 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.
Profile: Also known as ICC profile, this term refers to the mathematical representation of the way a device handles colours in relationship to a defined colour standard. Profiles underpin all colour management systems.
Resolution: The ability to reproduce fine detail. In digital imaging, this is generally defined by the number of pixels in the camera’s sensor or in the resulting image.
RGB: A colour model based on the red, green and blue components in the output, it is typically used for images that are captured by digital cameras and displayed on monitors.
Saturation: The intensity of a hue. Pastels have low saturation, while bright colours are highlysaturated.
Sharpening: An image enhancement technique that uses software to give more distinct edges to subject areas, lines and tones in an image.
TIFF (Tagged Image File Format): An image file format based on bitmapping that involves little or no data compression. Denoted by the *.tif extension.
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). USB 2.0 Hi-Speed, which is commonly used in today’s digital devices, can transmit data at up to 480 Mbits/second. USB 3.0, which was introduced in 2010, offers transmission speeds up to ten times faster. Special drivers are required to achieve high transfer rates.
See Printing Digital Photos Pocket Guide 6th Edition for more information.