Digital Imaging Dictionary

Digital Imaging terms explained

Whether you are a 'Newbie' or simply need a reminder we hope you will find this dictionary of terms useful.

© Finer Image.

A-F | G-M | N-S | T-Z

What is Dynamic Range? What is Colour Temperature?

Absolute Colorimetric

One of the 4 rendering intents in the ICC Profile Specification. Absolute Colorimetric Rendering is matching the colours in the destination color space exactly to the colors of the source color space. By definition this match is relative to the source colour space white point. Where a color is outside the gamut of the destination color space it is mapped to the nearest in gamut colour.

Acutance

Acutance is a measure of apparent Sharpness. It is not true sharpness because its possible to increase acutance (making an image appear sharper) without increasing actual resolution. Increased acutance is done by increasing the edge contrast in an image, typically by adding and thin light and thin dark line along edges. Photoshop's Unsharp Mask works on this principle.

Additive Synthesis

Additive Synthesis is the method of simulating many colours using a small number of primary colors, usually Red Green and Blue. The synthesis is said to be additive because pure Red Green and Blue light combined results in white light.

Adobe RGB

An RGB Colour Space developed by Adobe Systems. Often referred to as AdobeRGB1998 'cause this was the year they developed it :) Like sRGB it has a gamma of 2.2 but contains approximately 40% more colours (Gamut) than sRGB. Adobe RGB was intended to reflect the printable colours of common colour printers of the day.

Air Brush

Air Brush is the name of a small compressed air driven tool that can be used to spray atomized droplets of ink onto a surface. The technique was extensively used to alter photographs in the pre-digital era. In digital photography Air Brushing refers to digitally retouching an image in order to enhance its appearance.

Aliasing

Aliasing can be thought of as digital junk. Often when significantly down-sizing an image this 'junk' appears making the edges of text and images appear rough and pixelated. See also Anti-Aliasing

Alpha Channel

An alpha channel is a method of associating additional pixel related data with an image. In Photoshop alpha channels are used to store selections. The selections can be simple outlines or describe precisely how one image layer is blended with another.

Anti-Aliasing

Anti-aliasing is a digital filter applied to a digital image in order to minimize the effect of aliasing. The image is re-sampled via a mathematical algorithm in order to render the image more ideally

Aperture

An aperture is a hole or opening. In photography the aperture refers to the size of the hole that lets light through a lens. Variable aperture is used to control exposure and depth of focus in a photograph.

Artifact

A digital artifact is unwanted junk as a result of some digital process. A good example is the 'jaggies' that appear around heavily compressed JPEG images

Assigned Profile

Images don't always have an icc profile embeded in them but it is often known in what colour space they were created. In this case a profile can be assumed. The act of associating an ICC profile with an image is usually refereed to as Assigning a Profile. Often when resaving an image with an assigned profile, the profile is actually embedded into the image.

Assumed Profile

An ICC profile is not always embedded in an image but it is often known in what colour space the image was created. That being the case one can assume the profile of an image and render it's colours correctly. An example is images published on the internet where the sRGB profile is assumed. Of course if the profile is assumed incorrectly the rendering of colors in the image will be wrong.

Banding

Banding occurs where there is a breakdown in the smooth transition of tones in an image. Banding is characterized by a sudden change in tone with a visible edge rather than a smooth transition. It occurs because the image or output device has insufficient tones to create a smooth transition.

Barrel Distortion

Barrel distortion is a very common type of lens distortion. Instead of projecting the image with correct geometry barrel distortion causes straight lines to appear to bow outwards. This is most noticeable towards the edge of a projected image.

Bit Depth

Bit Depth or Colour Depth is the number of 'bits' used to describe the colour of individual pixels within an image. 'Bits' are the basic units of storage for a digital device. The more 'bits' used to describe a distinct colour, the more precisely that colour is described.

Bitmap

A Bitmap is a way of describing an image composed of Pixels. It's basically a grid describing the co-ordinates and colour of the pixels that make up the picture.

Bits Per Pixel

See Bit Depth

Black Point

Black Point refers to the darkest black that a device can produce. In Photoshop "setting the black point" refers to matching the darkest black in a digital image to the darkest black of the output device.

Black Point Compensation

Black Point Compensation was developed by Adobe and ensures that the maximum black of an output device is mapped to the maximum black of an input device during a colour conversion. This can be critical when converting from a CMYK colour space to an RGB colour space and vice versa. Without Black Point Compensation an image going from CYMK to RGB might not contain any true Blacks. Going the other way an RGB to CMYK conversion could easily result in clipping of dark areas of the image.

Bounce

A Bounce is used by a photographer to create a softer light and fill in harsh shadows when capturing an image. A bounce is literally a semi-reflective surface from which to reflect the main illuminating light (key light).

Brightness

Brightness describes the subjective response of a person to light intensity. Brightness perception varies from person to person because it is a response to an apparent light source not a property of the actual light source.

Camera Raw

Camera Raw files are built from unaltered image data captured by a camera's sensor. Camera Raw files contains more image data than would be evident in final JPEG or TIFF images. For example most digital cameras capture many more colours than can be represented by a JPEG in the sRGB colour space (a common output format for consumer cameras). Working with Camera Raw files is less convenient for the user because they require further processing but the final result can be far superior.

Canvas

Canvas refers to the 'Background Layer' of an image. A canvas can be the same size as the main part of the image but if a white or coloured border is required the canvas can be made larger than the main image area.

Chromatic Aberration

Chromatic Aberration is a lens distortion where colour fringes appear around objects. Chromatic Aberration is a failure of a lens to focus all wavelengths of light in the same place. It is most pronounced at boundaries between light and dark areas. It also tends to be more noticeable at the edge of images.

Clipping Path

A Clipping Path is a selection or alpha channel that identifies which parts of an image are transparent and which are visible. Selective clipping can allow parts of a background image to be seen through a foreground image.

Cloning

Cloning is reproducing a small section of an image elsewhere in the image. It is usually done when retouching an image to remove unwanted detail or to exaggerate existing detail.

Close Up Lens

A Close Up Lens is a simple secondary lens that is placed over a primary lens to reduce its focusing distance. This results in magnification of the projected image.

CMYK

CMYK (Cyan Magenta Yellow Black) is a four component model for describing colour. Each colour within the CMYK model is described as proportions of C,M,Y & K. CMYK is used largely in printing and is known as a subtractive colour model - the less of each colour printed, the closer you get to white (white paper).

Color Balance

Colour Balance is the relative mix of component colours (e.g. Red Green & Blue) Adjusting Colour Balance is one way to make an image look perceptually correct. Colour Balance adjustments are usually done to remove global colour casts from images.

Colour Depth

See Bit Depth

Colour Management

Colour Management is the name given to the science and process of maintaining accurate colour between various input and output devices. The goal is to capture or render colour in a consistent way no matter what the device.

Colour Rendering Index

Colour Rendering Index is an objective measure of the ability of a light source to reproduce the colours of objects faithfully in comparison to natural daylight. High CRI lights are essential for Print Viewing and Assessment

Colour Space

A colour space is a range of colours (gamut) described in absolute terms with the use of a colour model. It is a sub-set of all possible colours. The absolute reference is usually a standardized description of colour such as CIEXYZ but can be any absolute reference such as a pantone colour swatch.

Colour Temperature

Colour Temperature is a description of the colour of light when compared to the temperature of an 'ideal black body'. Lower temperatures are redish and higher temperatures blueish.

Crop Factor

Sometimes referred to as magnification factor. When a lens designed for a larger format is used on a digital camera with a sensor smaller than the 'larger format' some cropping occurs. This is because the sensor is recording only the central portion of a larger projected image. The image appears magnified because the central portion of the image is spread out across all the pixels of the sensor.

D-Max

D-max is the maximum density that can be achieved by a material (such a ink on paper). The higher the D-max, the less light is transmitted and the blacker the material will appear

D50

D50 is a "Standard Illuminant" as defined by the Commission internationale de l'Eclairage (CIE). It describes the Spectral Power Distribution (SPD) of natural daylight at 5003 K.

D65

D65 is a "Standard Illuminant" as defined by the Commission internationale de l'Eclairage (CIE). It describes the Spectral Power Distribution (SPD) of natural daylight at 6504 K.

Digital Image

An image composed of binary data (ones and zeros) usually created by a computer or other binary device. Most digital image are composed of elements known as Pixels but are created using vectors.

DPI

DPI or Dots Per Inch is a printing term. It refers to the number of dots a printer makes in each direction. So a printer that outputs at 300 dots per inch will put down 300 x 300 dots per square inch. That's 900,000 dots

Dye

A Dye is a coloured substance that is absorbed into or chemically reacts with the material to which it is applied. This is quite different to a pigment that does not react with but instead coats the material.

Dynamic Range

When applied to digital imaging, Dynamic Range is the ratio of the whitest white to the darkest black distinguishable and recordable on a device. For a digital camera this is a measure of the contrast range the sensor can capture. For a printer this is the difference between paper white and the most dense printable black.

Embedded Profile

An embedded profile is an ICC Profile that has been saved onto the data of an image. JPEG and TIFF images are examples of image formats that allow ICC profiles to be embedded.

Exposure Bracketing

Exposure bracketing is the process of taking multiple pictures of the same scene using varying exposures. Say from 1 stop under exposed to 1 stop over exposed in 1/3 stop increments. The techneque is very useful for tricky exposures of contrasty scenes. During post processing the photographer will choose the best exposure.

Gamut

A Colour Gamut is a range of destinct colours. Often Gamuts are displayed as 3 dimensional plots of colour. In this way the measured gamuts of various output devices can be compared.

Gamut Warning

If a colour is outside the set of colours that a device can reproduce it is said to be "Out Of Gamut". Photoshop has a Gamut Warning feature that highlights the parts of an image that cannot be reproduced on the output device.

Grayscale

Grayscale is a fancy way of saying Black and White. The pixels in a grayscal image are described only in terms of intensity rather than intensity and colour as is the case with coloured images

Highlight Clipping

When lightening an image it is possible to cause near white tones to become white, resulting in a loss of highlight detail. This is known as 'clipping the highlights' or 'highlight clipping'

Interpolation - Bicubic

Bicubic Interpolation is used in digital imaging when resizing an image to create a new set of pixels from the initial set. Bicubic Interpolation is particularly good for very detailed images such as photographs

Interpolation - Bilinear

Bilinear Interpolation is used in digital imaging when resizing an image to create a new set of pixels from the initial set. In most cases Bicubic Interpolation is better because it use more data points to calculate the new pixel values

Interpolation - Nearest Neighbour

Nearest Neighbour Interpolation is used in digital imaging when resizing an image to create a new set of pixels from an initial set. Nearest Neighbour interpolation gives best results for line art or simple shapes

JPEG

JPEG stands for Joint Photographic Experts Group. JPEG provides a way of compressing images to save storage space. A user can select the amount of compression but the more the compression the lower the image quality because image data is lost in the process.

JPEG2000

JPEG2000 is a technical advance over JPEG that was intended to superceed JPEG but it doesn't enjoy any where near the widespread support of JPEG.

Kelvin

Kelvin is the SI unit of temperature even though most of us use Celcius or Fahrenheit. In digital imaging it is used to express Colour Temperature

Magnification Ratio

Magnification Ratios describe the size of an image projected through a lens relative to the size of the subject being projected. In digital photography the size of the projected image is measured at the digital sensor. A ratio of 1:1 is considered true Macro magnification.

Mega Pixels

1 Mega pixels = 1,000,000 pixels

Out Of Gamut

When a Colour defined in a Source Colour Space falls outside the range of colours defined in the Destination Colour Space, that Colour is said to be " Out Of Gamut". In practical terms this means it cannot be reproduced by the output device.

PDF

Portable Document Format. An open standard for saving,reading and exchanging documents irrespective of operating system. A PDF document often contains text and images along with detailed descriptions of how those elements should be rendered. Fonts and colour profiles can be embedded in the document.

Pixel

A PIXEL is a PIcture ELement. If you're wondering about the X it because PIXEL is really the joining of the words PIX and Element where PIX was a popular slang for Pictures. As in - "We bagged some great pix from opening night!"

PPI

PPI or Pixels Per Inch is a simple measure of the density of pixels in a bitmap image at ouput dimensions. For example an image prepared for printing might be 300 PPI at an output dimension of 8 X 10 Inches. This means in containes 300 X 8 X 300 X 10 = 7,200,000 pixels (7.2 Meg Pixels)

PSD

PSD is the file extension for Photoshop Documents. Multi-layered Photoshop documents have much smaller file sizes than their multi-layered TIFF counterparts.

RGB

RGB stands for Red Green Blue and refers to a colour model in which colours are expressed in amounts of Red Green and Blue light. RGB is known as an additive model of colour as adding Maximum Red Maximum Green and Maximum Blue light produces white.

Shadow Clipping

When darkening an image it is possible to cause near black tones to become black, resulting in a loss of shadow detail. This is known as 'clipping the shadows' or 'shadow clipping'

Sharpness

The Sharpness of a photographically captured image is a function of the resolving power of the lens and the resolving power of the image capture technology (i.e. film or image sensor). Human perception of sharpness is also affected by the Acutance of an image

Soft-Proofing

Soft-Proofing is a method of accurately simulating on a display monitor what a document or image will look like when printed. Soft-Proofing relies on having Custom Printer Profiles and Monitor Profiles

Video Card

An electronic component commonly found in computers with the function of generating output images on a display. The output from a video card is adjusted as part of a Monitor Calibration

White Point

White Point describes the colour temperature of a 'white colour' captured or produced by a device. For example when the RGB colour control signal 255,255,255 is sent to a monitor, the 'white colour' produced will have a tint that can be expressed in terms of colour temperature.