Photographic Glossary-G


G curve
The average gradient of a characteristic curve, describing similar characteristics to gamma, but measuring the slope from a line joining the lower and upper limits of the curve actually used in practice.

G4 Compression
A compression technique used in Fax Group 4. It produces very good results for black and white, and is frequently used as an option in TIFF files for black and white images. It is also used in Adobe Acrobat (PDF) files.

Gain & Level
Gain and level are image processing terms which roughly correspond to the brightness and contrast control on a television. The gain is the " contrast " and the level is the "brightness". By changing the level the entire range of pixel values are linearly shifted brighter or darker. Gain on the other hand linearly stretches, shrinks the intensity range, thus altering the contrast.

A method of adjusting a CCD sensor's sensitivity to light.

Gamma correction
The measure of contrast that results in lightening or darkening the midtone regions of an image. Also, the amount of midtones need to be adjusted on a monitor.

The contrast affecting the mid-level grays or midtones of an image. Adjusting the gamma of an image allows you to change brightness values of the middle range of gray tones without dramatically altering the shadows and highlights.

Gamut Compression
The editing of an image to reduce the colour gamut so that the image can be displayed or output within the limits of a particular device.

Gamut Mapping
The plotting of an image colour gamut into the CIE colour space.

The range of colours and tones a device or colour space is capable of recording or reproducing. The human eye can sense many more colours than can be reproduced on a computer monitor in RGB colour space.

Putting a group of images or jobs on the scanner or press at one time.

A computer server that allows for the connection of different computer network using protocol conversions.

Gaussian Blur
An image softening effect utilizing a bell shaped Gaussian distribution to apply the softening effect.

Abbreviation for gray component replacement; the separation technique where black ink is used to replace either a portion of the unwanted component in a saturated colour, or a combination of cyan, magenta, and yellow equivalent to the unwanted component. Typically specified to improve colour control on older presses.

Gelatin filter
Are filters cut from dyed gelatin sheets and held in front of the lens or studio light.

A substance produced from animal skins and bones, it is the basis for modern photographic emulsion. It holds light-sensitive silver halide crystal in suspension.

General protection fault ( GPF )
A general protection fault error message occurs when an application tries to write to memory that is not designated by the operating system.

Generation loss
The loss of quality that occurs in any type of analog duplication such as an interneg. Digital images do not have this kind of loss of quality when duplicated.

Ghost image
In time exposure photography, an object that is only partially recorded on the film and therefore has a translucent, ghost-like appearance. Some people also refer to " flare " as a ghost image. Bright spots of light, often taking the shape of the aperture, which appear in the camera viewfinder or in the final photograph when a lens is pointed at a bright light like the sun; controllable through the use of multilayer coating of the lens elements.

GIF 89a
The most recent GIF standard that allows the selection of area for transparency. The primary use is on the Internet and other on-line services. Like GIF it is 256 colour or 8 bit imaging. A transparent GIF (Graphics Interchange Format) is an image file that has one colour assigned to be "transparent" so that the assigned colour will be replaced by the browser's background colour, whatever it may be. As an example, if you have created a rectangular GIF image of a large blue square on a white background but are only interested in having the blue square appear on your Web page, and don't want to see the white background, you can make the white background colour transparent so that it changes to whatever the Web page's background colour is (yellow, for example). Then, when you view the Web page, you will only see a blue square on the yellow background.

Graphic Image File format. A widely supported image-storage format promoted by ComputerServe that gained early widespread use on on-line services and the internet.

A measure of computer memory or disk space consisting of about one thousand million Bytes ( a thousand megabytes). The actual values is 1,073,741,824 bytes (1024 megabytes).

Describes a printing paper with a great deal of surface sheen. Opposite: matte.

GN (Guide number)

Used to express the power output of the flash nit. It indicates the power of a flash in relation to ISO film speed. Guide numbers are quoted in either meters or feet. Guide numbers are used to calculate the f/stop for correct exposure as follows: Number calculated by multiplying proper flash exposure aperture by the subject distance.

Gallium Photo Diode. Metering cells for measuring exposure, using gallium arsenide-phosphide, just like SPD or Cds cells.

A smooth transition between black and white, one colour and another, or colour and no-colour.

Gradient fill
An image fill that gradually transitions from one colour to another; commonly used in graphics editors.

Minute metallic silver deposit, forming in quantity the photographic image. The individual grain is never visible, even in an enlargement, but the random nature of their distribution in the emulsion causes over-lapping, or clumping, which can lead to graininess in the final image. Also cross check with below for graininess. When a photographer talks about fine grained films, they are talking about the smallest distinguishable component of a print. The slower the ASA rating, the finer the grain, and the converse is also true.

Graininess ( Grain )
The granular appearance of a negative, print, or slide. Graininess becomes more pronounced with faster film and the degree of enlargement. The sand-like or granular appearance of a negative, print, or slide. Graininess becomes more pronounced with faster film and the degree of enlargement.

Gray balance
The balance between CMY colourants required to produced neutral grays without a colour cast.

Gray card
A card that reflects a known percentage of the light falling on it. Often has a gray side reflecting 18 percent and a white side reflecting 90 percent of the light. Used to make accurate exposure meter readings (meter base their exposures on a gray tone of 18 percent reflectance) or to provide a known gray tone in colour work.

Gray level
A shade of gray assigned to a pixel. The shades are usually positive integer values taken from the gray-scale. In a 8-bit image a gray level can have a value from 0 to 255.

Gray scale
An image type that contains more than just black and white, and includes actual shades of gray. In a grayscale image, each pixel has more bits of information encoded in it, allowing more shades to be recorded and shown. 4 bits are needed to reproduce up to 16 levels of gray, and 8 bits can reproduce a photo-realistic 256 shades of gray.

Grayscale image
An image consisting of up to 256 levels of gray, with 8 bits of colour data per pixel.

An acronym for Grey Component Replacement, a technique for reducing the amount of cyan, magenta and yellow in the mid tone areas and replacing them with the appropriate level of black.

Grey card (18% Grey Card
Tone used as representative of mid-tone of average subject. The standard grey card reflects 18 per cent of the light falling on it.

To make a strobe light to appear directional, like the light of the sun, a grid is used. It is basically a black honeycomb pattern that is placed in front of the flash head to cause a directional effect and minimize spillage of light causing a diffusion effect. Also called a Gobo or a Cookie.

Ground glass
Frosted glass used as a viewing mechanism in cameras without prisms. The glass is placed so that the lens projects the image against the glass for focusing and composition purposes.

Pronounced "Gooey", stands for graphic user interface. Refers to the computer interface with software in a user friendly appearance.

Guide number
A number used to calculate the f-setting (aperture) that correctly exposes a film of a given sensitivity (film speed) when the film is used with a specific flash unit at various distances from flash to subject. To find out the f-setting, divide the guide number by the distance.

Guide Number
Most flashguns for a camera list a rating referred to as a Guide Number, or GN. This number is used for manual settings to determine the proper aperture based on the distance from the subject. Guide numbers are also rated assuming ISO 100 speed film. If your flash has a guide number of 120 and the subject is 12 feet away, chose the aperture closes to 10 (120/12=10), which would be f/11. If the subject was 20 ft away, the result would be close to 6 (120/20=6), or f/5.6. This is all under the assumption that 100 speed film is being used. If 400 speed film was being used, which is 2 stops faster film, meaning it requires less light to expose the film, the aperture would be stopped down 2 stops, from f/5.6 to f/11. Instead of 400, if 200 was being used, then the subject do camera distance would require f/8.

Gum Bichromate
Often called "gum." An early process in which exquisite colored prints are made by printing on paper coated with layer(s) of sensitized and pigmented gum arabic.

A recently discovered process which has the look and feel of some of the ancient processes. In combination with unpigmented gum, etching bleach and oil pigments, it is possible to build monochrome or polychromatic images.