Photographic Glossary

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It was suggested that we should put together a photographic glossary of useful terms. I have taken on the task of creating this list and have great pleasure in posting part 1. Part 2 to follow below.

AdobeRGB
Colour space recommended by Adobe Systems, Inc. for commercial printing. It offers a wider range of colour reproduction than sRGB. When images using AdobeRGB are opened by non-compatible software, the colours look lighter.

Auto Exposure (AE) Metering
The brightness of the subject is measured by the camera and these measurements are then used automatically by the camera to set the exposure. Some typical metering modes are Multi segment Metering, Centre weighted metering, and Spot Metering.

Aperture
The “hole” in the lens that lets the light through. The aperture increases or reduces the size of the light beam that comes through the lens. The bigger the hole, the more light comes through. See also f-number.

Aperture Priority
A mode of operation where the photographer has control over the aperture of the lens. The photographer manually selects the aperture and the camera then automatically sets an appropriate shutter speed to give the correct exposure. This is most often used when the photographer wishes to control the depth of field in the resulting picture.

Auto Bracket
A mode of operation that automatically changes the exposure of the picture being taken. Typically, when the shutter button is pressed, three images are shot. The first one has no compensation, the second is under exposed and the third is over
exposed. The order and extent of under/over exposure can usually be varied.

Camera Shake (Blur)
If the camera moves while the shutter is open, the entire image appears blurred. This occurs more often when shutter speed is slow. Some ways to reduce camera shake are:
using the flash, raising the shutter speed or alternatively, use a tripod/monopod to stabilize the camera. Because camera motion causes camera shake, if your (SLR) camera has a mirror lock-up facility, this can be used to reduce blur. As a general rule, if you are an dSLR user, the shutter speed should not be lower than 1/(focal length of lens x digital sensor crop factor). If you are using a camera with a 1.5x crop factor and a 50mm lens, the shutter speed should ideally not be set to less than 1.5x50=1/75th of a second in order to minimise blur.

Centre Weighted
A metering mode where the camera measures the light entering the camera and is strongly influenced by the amount of light in the centre of the image. It measures the light intensity over the whole image but biases the exposure to try and get the centre of the image correctly exposed.

Colour space
A defined range of colours from the spectrum which are used. In digital cameras, sRGB is defined as the standard by the Exif format. AdobeRGB is also used, generally for commercial printing.

Colour Temperature
The colour of the light source illuminating the subject. This is usually indicated in absolute temperature, using the Kelvin (K) scale. The colour of light shifts to a bluish colour as the colour temperature rises, and to a reddish colour as the colour temperature falls.

Crop Factor
The sensors used in dSLR are often a different size (smaller) to a 35mm film frame. The focal length of the lens being used determines the size of image that the lens projects onto the sensor. As the digital sensor is sometimes smaller than the film frame, the lens only projects the middle part of the image onto the sensor and the outer parts of the image “fall off the edges” and are lost. This gives the effect of using a longer focal length of lens. Typical digital sensor crop factors are 1.5x or 1.6x. A 50mm lens used on a digital camera with a 1.5x crop factor will give a similar view to a 75mm lens used on a 35mm film SLR.

Depth of Field
The area of acceptable focus extending in front of, and behind, the exact point of focus of the lens. Reduce the aperture (use a bigger f-number) to increase the depth of field.

EV (Exposure Value)
Traditionally used on hand-held exposure meters. The Exposure Value is a figure used to represent a particular intensity of light. The measured EV can be used to determine a particular shutter speed and aperture setting. A given EV can be satisfied by a large number of equivalent shutter/aperture combinations. See the “manual exposure, juggling three balls” sticky thread http://www.worldphotographyforum.com...read.php?t=125 for more explanation. Exposure Values may typically range from EV1 (satisfied by 1 second, f/4, ISO 800) to EV18 (satisfied by 1/4000th second, f/5.6, ISO 50).

Exposure Compensation
The process of changing the shutter speed and aperture value to vary from the figures suggested by the camera’s meter measurements. A camera’s meter is calibrated to give the correct exposure for a mid tone grey surface for a given amount of light being reflected from it. The camera does not know if the reading it has taken is due to a large amount of light being reflected from a very dark surface, or a small amount of light being reflected from a light surface. The camera will hedge its bets and plump for grey in either of these extreme situations. Exposure compensation is used to adjust for this kind of situation.

EXIF (EXchangeable Image file Format for digital still camera)
A standard digital camera file format established by the Japan Electronics and Information Technology Industries Association (JEIDA).

f-number
The number resulting when the focal length of a lens (in mm) is divided by the diameter of the aperture (effectively how big the hole is in mm). The sequence of f-numbers is used to calibrate the aperture in regular steps, known as f-stops. F-numbers traditionally followed a sequence where each stop represented a halving or doubling of the amount of light the aperture would let through. Modern lenses will often now increment in one half or one third stop steps. As the f-number is a ratio of focal length to aperture, the f-number becomes progressively bigger as the aperture becomes smaller to allow in less and less light. A lens with a small f-number is often called a “fast” lens because it allows a faster shutter speed to be used for any given Exposure Value.

Histogram
A graph that shows the darkest and brightest points in an image. The horizontal axis represents the brightness and the vertical axis represents the number of pixels present in the image at that particular brightness.

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Hot Spot
Areas in an image where the sensor has reached the maximum amount of brightness it can register. These areas lose contrast and appear white.

JPEG
An image compression method. The amount of image compression can usually be varied in the camera or when saving files on your PC. A higher JPEG compression factor will result in a given file having a smaller size, thus aiding storage but the compression will result in flaws in the image which may be visible when viewed.

Multi-segment metering
In this form of exposure metering, the camera takes measurements from a range of different points in the image and evaluates all the different light meter readings. It will then choose camera settings to give what it thinks is the best compromise for the correct exposure over the whole image.

Noise
Image roughness or unevenness, usually visible as areas of faint random colours seen against the intended background colour. Often caused by particularly low shutter speeds. Compact cameras and “superzoom” model cameras will suffer from noise more than dSLR models as compact/superzoom models have more pixels crammed into a smaller area on the sensor. This higher density of pixels causes interference between the adjacent pixels and is often the source of noise.

Prime Lens
A lens with a fixed focal length. Usually “faster”, i.e. capable of a wider maximum aperture, than a zoom lens set to the same focal length. Almost invariably capable of better optical resolution than a zoom lens set to the same focal length. Lacks the flexibility of a zoom lens.

Program Mode
A mode of operation where the camera automatically selects both the shutter speed of the camera and the aperture of the lens to give what it thinks is the correct exposure. Sometimes referred to as “Point and Shoot”.

RAW data
Unedited image data output from the camera’s sensor. Raw data is data before it has been internally processed by the camera. In addition, RAW data is 12bit data that contains 16 times the information of 8bit JPEG and TIFF data. It is possible to correct bigger errors in exposure or white balance if the RAW data is available to be worked on. RAW data needs processing using the software provided by the camera manufacturer or a third-party application such as Adobe Camera Raw before it can be viewed by standard picture viewing applications.

Sensitivity
Usually measured as an ISO “film speed” value typically between 50 and 3200. Each doubling of the number, e.g. from 200 to 400, doubles the sensitivity meaning that half the amount of light is needed to obtain the correct exposure. With a high sensitivity. images can be shot with a high shutter speed even in dark places, this can be used to reduce camera shake. However, images with high sensitivity are more susceptible to noise.

Shutter Priority
A mode of operation where the photographer has control over the shutter speed of the camera. The photographer manually selects the shutter speed and the camera then automatically sets an appropriate aperture to give the correct exposure. This is most often used when the photographer wishes to control the amount of motion blur in the resulting picture. Motion blur can be unintentional due to camera shake, or intentional to convey a sense of movement. A faster shutter speed will have a greater tendency to freeze the motion of the subject, i.e. reduced motion blur.

Shutter Speed
Measured in seconds, it is the length of time that the shutter is open and allowing light through to the sensor. Altering the shutter speed changes the amount of light that strikes the sensor. The shutter speed is proportional to the amount of light allowed through to the sensor, i.e. doubling the shutter speed from 1/125th of a second to 1/250th of a second halves the amount of light the sensor receives.

Spot Metering
A light metering mode where the reading is taken from a very small “spot”, usually in the centre of the image. Usually spot metering is done so that the spot metering point is the part of the image the photographer wants to be correctly exposed and there are other areas in the image that would otherwise confuse the metering. Exposure Compensation may still need to be applied.

sRGB (standard RGB)
International standard of colour space established by the lEC (International Electrotechnical Commission) This is defined from the colour space reproducible on PC monitors and is also used as the standard colour space for Exif.

Vignetting
Usually refers to darkened corners on an image. Vignetting can occur when a flashgun has not been set to a wide enough angle to cover the image in view. Vignetting can also occur if a lens designed for a digital sensor is used on a film camera body (see Crop Factor).

White Balance
The colour temperature and tint of an image is adjusted to match the light source so that the subject appears be the correct colour. This is known as setting the White Balance point, you are defining what is “white” for a given light source. A range of typical White Balance settings on a camera could be daylight, shade, cloudy, fluorescent light, tungsten light or flash.

Zoom Lens
A lens which can be adjusted over a range of focal lengths. Usually “slower”, i.e. not so capable of a wide maximum aperture, than a prime lens of the same focal length. A compromise on optical resolution compared to a prime lens. Has flexibility as one lens can be used in place of a range of different prime lens.