Glossary of Acronyms, Digital & Photographic Terms

A

Acrobat
Defines a file created in Adobe Acrobat 'Portable Document Format" or PDF. PDF files typically capture the type style employed, simplified colour, size of page and whether raster or vector graphic files. By defining each object in mathematical terms, as does the PostScript printing language, a file can be compressed in size with little apparent loss in quality. At the same time it becomes platform independent – capable of use by Mac and Windows users alike. To create an Acrobat file, the application program (i.e. Adobe Photoshop, Adobe InDesign etc) must include a licensed Distiller component, to save the regular file as an Acrobat PDF file. Today one can also find programs from other vendors that create Acrobat files from Word or other business documents.

Additive Colour
The basic principle of adding the three primary colours, red, green and blue, to create all the other colours in the spectrum. Additive colour is used in monitor displays using combinations of red, green and blue phosphors to combine to produce multi-colour displays.

Adjustment Layers
Often you want to experiment with colours when retouching images. An adjustment layer can be set up in programs like Adobe Photoshop or Corel PaintShop Pro to allow you to try such changes, store the interim adjustment, but come back to the original later if it didn't work out. Adjustment layers can also be used to change relative brightness and contrast of selected items.

ADSL (Asymmetric Digital Subscriber Line)
A high-speed connection service over a standard twisted-pair copper wire telephone line, sometimes referred to as a DSL line. Download speeds range from 500kb p/sec - 6Mb p/sec. Upload rates are lower and vary between 64Kb/sec and 500Kb/sec. This compares to a dial-up modem connection speed typically between 40 and 50kb/sec connection speed and typical download rates of between 4 and 6kb/sec. ADSL2 is the recently emerging higher speed system available over regular copper wire phone lines to your home or business.

Alpha Channels
When you want to manipulate a specific part of an image when editing it for retouching purposes, an alpha channel can be created to store a selection as 8-bit greyscale images. These are added to the colour channels in an image. An RGB (Red Green Blue) image has three default channels, one for each colour, together with a composite channel for editing the entire image.

Ambient Light
Light that is derived from natural light within a scene. Sometimes called 'existing light', this is the kind of lighting preferred by photojournalists to avoid the often flattening effect, or the artificial lack of contrast, produced by an on-camera flashgun.

Anti-Alias
Anti-aliasing smoothes the edges of objects and the transitions between them. It is used when displaying type on a computer monitor to smooth the otherwise rough edges of rounded objects when displayed in square picture element displays. Anti-aliasing is also used when laying one object cut from one background colour and superimposed over another. An anti-alias filter is also employed over CCD sensors in digital cameras to integrate the R, G and B pixels.

Applet
Applets are small applications in the Java programming language developed by Sun MicroSystems, that run within other applications. Typically they are found in more complex Internet pages. When you click on certain sections, an Applet or mini-program running on the network server, will launch a sequence of program strokes to deliver a solution to your query or request.

Archived Images
Pictures that have been stored and properly indexed for later retrieval, either on CD-ROM , DVD or hard drives in digital form, or on acid-free paper, printed out either photographically or via pigment-based inkjet printers.

Artifacts
Artifacts are image elements introduced in recording or reproduction of images, which were not there in the original. Typically they are introduced when excessive compression is applied, or a file of too small a size is scaled up or re-sampled through interpolation. In a JPEG image file, artifacts are pixels of colour "guessed" by the decompression algorithm.

Aspect Ratio
The relationship in size between one dimension of an image and the other. A compact digital camera will typically be a 4:3 ratio, whereas the 35mm frame is 3:2. Older film aspect ratios were 6:7, 6:4.5 and 6:6 (medium format cameras) and full- and half-plate film images of 10:8 and 5:4 respectively. The horizontal dimension is expressed first.
      Increasingly common is the HDTV or widescreen format of 16:9 and found in WXGA monitors on laptop computers. The DSLR format adopted by Olympus uses the 4:3 format and lenses for such cameras have a 2x multiplier effect over their 35mm film camera equivalent.

Aspherical
Non-spherical glass elements used in a complex zoom lens, usually molded to an adjacent spherical lens (i.e. one that has a constant radius). Aspherical lens elements allow lighter weight and more compact lens design. The Tamron 18-250mm lens, for example, employs ASL lens elements.

AVI
The Microsoft-developed video and audio file format most commonly found for video material within multimedia applications. The WAV file is the equivalent sound-only file format.

AWB (Auto White Balance)
An auto-exposure system in a digital camera or video camera which correctly interpret white subjects and so correctly interpret colours in the colour spectrum. This alleviates (as much as possible) colour casts within the picture.

B

Backpack
A camera bag designed to be worn on the back, that has provision in many cases for a laptop computer and separate, sometimes waterproof compartments, especially contoured to protect photo equipment.

Banding
A visible stepping of hues in a gradient of colour information, such as a continuous tone image. This may appear to be an artifact in a medium-resolution scan, for example.

Barrel Distortion
In an image, the convex rounding of a square or rectangular image caused by lens design compromises. The opposite term is pincushion distortion which pushes straight lines into the centre of an image, most commonly found in long zoom lenses at the telephoto end of the zoom range. (These phenomena are minimised in lenses of a fixed focal length.)

Bayer Filter
Named after Dr Bryce Bayer, who patented his colour filter array for Red, Green and Blue patterns of filtration on a square photo sensor grid, originally for CCD image sensors. Since a CCD is essentially colour-blind, the sensor needs to be given instructions as to how to code different light brightness and reflections in order to create a colour image.

Bitmap or BMP
A grid of small squares or pixels used to define an image. As the image is scaled up the size of each grid or pixel is enlarged, eventually becoming visible, resulting in a jaggy appearance. However, to reproduce subtle gradations in tone and colour, when the size of the reproduced image is known at the time of file creation, a bitmap or .bmp-type file can be very effective, especially for electronic presentations.

Black Point
The darkest shadow in the levels of an image histogram, used to set the dynamic reproduced range of an image.

Blooming
Digital cameras using CCD sensors can create false contrast in transitions between light and dark portions of an image. The 'halo' most often seen is a purple fringe, something which exists to a greater or lesser extent in all digital cameras. It is caused by excess charge in the pixels immediately adjacent to the transition in brightness. Programs like Adobe Photoshop CS3 have special tools to remove these halos.

Bracketing
A quick sequence of images taken by a photographer when precise exposure can only be estimated, to ensure an image of the right density is recorded. This can also be used to enable highlights and shadows to be shot in different images that can later be merged for a printable or viewable result.

Buffer Memory
The inbuilt memory in a digital camera, scanner or printer that temporarily stores captured data, prior to full file-saving or execution in the device. High-megapixel cameras depend on large 40–50MB buffer memory to enable rapid continuous shooting of 8 fps or greater, up to 60 frames or more.

Burning In
Using a special “brush” to selectively darken part of an image. Photoshop includes tools to burn in an area, or remove brightness. The opposite to burning in is called dodging – selectively lightening part of the picture. Both terms are derived from standard film enlarging techniques, where a negative was projected down onto light-sensitive paper. The emulsion in this was also a negative, so the combination of film and paper produced a positive image. To darken a part of an image, therefore, that portion was 'burned in', given extra exposure to light; to make it lighter, it was blocked from the light, or 'dodged', while the rest of the image area was exposed.

Byte
The measurement of digital data equivalent to 8 bits. 500 bytes is equal to 4 kilobits. A download speed of 6kb/sec would be typical of most modem-connected computers.
      When abbreviated in kilobytes or megabytes, common use is to express these as KB and MB respectively. Kb and Mb refer to kilobits and megabits. 1Kb is one eighth of the size of a 1KB or kilobyte.

C

Cache
Local temporary data, often held in memory, which is quickly available for use when using a program. When using the internet, pages and objects will be saved to your hard disc 'Internet Cache' file. When you next visit the same site, your computer draws the image or text file from your computer instead of downloading it again. To ensure you always see the latest page information, it is an idea to refresh or reload your page, at which time your browser will re-search the internet for the latest information and load that instead.

Calibration
Setting up a series of imaging devices to produce predictable outcomes. Probably the most imprecise part of the imaging process. Ideally the image that you capture on your digital camera or scanner will display on your monitor and accurately reproduce the colours and tones of the original. When printed, a calibrated system will produce an image in the colour and intensity expected. A monitor display uses RGB (Red Green Blue) colour space at varying degrees of brightness (luminance) and colour intensity (chrominance or chroma). A commercial printer uses CMYK colour inks (Cyan Magenta Yellow Black). Since they represent quite different levels of colour and brightness, calibration of your imaging system is required to get an approximation of the same colour space at each step.
See also Colour Calibration.

CCD (Charge Coupled Device)
A light detection silicon device employed in scanners and digital cameras, both still and motion. It produces an electric current in proportion to the amount of light that strikes each CCD element or pixel.
See also CMOS.

CD-ROM (Compact Disc, Read-Only Memory)
A compact disc with computer data recorded on it, which can only be read. It is, in effect, read-only memory.

Channels
The locations for colour information when stored. An RGB image has three colour channels and one composite channel. If spot colour is added, an additional channel is added. If a selection or series of selections are made and saved, then these are additional channels, usually known as Alpha channels.

Cloning
The selection of a particular area of an image painted into another area of the image, either to correct a defect, i.e. a scratch or artifact, or in image restoration where part of an image may be torn or actually missing. This feature can be found in programs like Adobe Photoshop or Photoshop Elements .

CMOS (Complementary Metal-Oxide Semiconductor)
Digital cameras have now become available with these sensors. Initially they were best suited to low-quality VGA-sized capture devices. but more recent designs offer up to 16-megapixel and even 44-megapixel sensors. At those levels they become harder to manufacture with consistent yield, but results can be very 'photographic' in appearance, which some people describe (incorrectly) as soft.

CompactFlash
A type of flash-memory card used today in professional digital cameras and handheld computer devices to store images or programs. CompactFlash cards currently vary in capacity from 128MB to 16GB, and now even 64GB. They are available as Types I (3.3mm thick) and II (5mm thick) and IBM's miniature-sized disc drive, with capacity to 4GB, branded a Microdrive. A MicroDrive fits into the thicker Type II slot. It generates substantially more heat than flash memory, while also tending to greater fragility.

Compression
Digital cameras using JPEG (Joint Picture Experts Group) compression convert the image into compressed form prior to storing the data. When the file is re-opened a mathematical algorithm puts back the data compressed out when saving the file. In a continuous tone sky, the compression algorithm will say "OK, reproduce this pixel colour 200 times for the next 200 pixels, encode that pixel with that instruction and save the data space of 200 identical pieces of data. Errors in the JPEG formula lead to introduction of artifacts or false data when reproduced.

Continuous Burst
The uninterrupted period in which a digital camera can continue to shoot images before the buffer memory within the camera is full and prior to the temporary images being offloaded into the flash memory device, such as a memory card.

Contrast
The difference between the lightest and darkest parts of a scene. In greyscale (black-and-white) imaging terms, the largest contrast is between 0 and 255 shades of grey, representing pure white to complete black.
      A low-contrast image has little difference between the lightest and darkest tones, with a lot of grey tones in between; a high-contrast image has few mid-greys and a lot of very light and very dark areas.

CMYK (Cyan Magenta Yellow Black)
The three subtractive primary colours (C, M & Y) combined with black (K) for print output. Traditionally used in offset printing production (e.g. magazines and newspapers), but now also in home inkjet printing. Some inkjet printers will contain 'lighter' C, M & Y inks as well, requiring up to 7 cartridges instead just the usual 4. These are designed to give superior reproduction in the highlights where darker inks may not be capable of rendering the tones.

Colour Calibration
The process of managing the differences in colour space between different imaging devices. Devices sometimes come with ICC (International Colour Consortium) profiles and programs like Adobe Photoshop can interpret these profiles for consistent output. Other colour management systems are Apple's ColorSync, Kodak Digital Science CMS and Microsoft ICM.
      Use of recognised colour profiles enables image files to be used across platforms and devices and continue to reproduce in a predictable way. Colour consistency is also only achieved where the same colour space is employed in the capture and processing function, such as RGB, sRGB, or Adobe 1988 etc.

Cropping
Cutting away part of an image to tighten up the subject or to improve layout within a picture.

Compiled, written and updated by John Swainston.
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