Absorption The blue, filtering effect of sunlight. Water absorbs the colours of the spectrum selectively until, at depths of around 20—25 m, only tones of blue and green remain.

AE lock (AE-L, Auto Exposure Lock) This enables you to take a light-meter reading from part of an image and then hold that setting while you compose the picture.

AF lock (AF-L, Auto Focus Lock) This enables you to lock an auto focus lens in its present focus setting and then hold that setting while you recompose the picture.

Ambient light (also referred to as natural light) The light from the sun which is available underwater.

Angle of view The extent of the view taken in by the lens, which varies with focal length for any particular format size. The angle made at the lens across the image diagonal.

Aperture Behind the lens of your digital camera is a circular iris that opens and closes to determine the amount of light falling on the CCD. Altering the aperture also changes the depth of field.

Aperture preview A small button close to the lens on some SLR cameras that allows you to visually check the depth of field in the viewfinder.

Aperture priority A semi-manual exposure mode which allows the user to set the aperture according to the depth of field required, while the internal metering system selects the appropriate shutter speed to obtain the correct exposure.

Apparent distance The distance at which objects appear to be away from the eye/camera. Objects underwater appear to be one-quarter closer than they really are.

Artefacts Compressing an image sometimes causes noise to appear as angular blocks or artefacts.

Auto Focus (AF) System by which the lens automatically focuses the image of a selected part of your subject.

Automatic An exposure mode found on digital cameras in which all the camera settings, including ISO, white balance, shutter speed and aperture, are chosen by the camera. This is useful for beginners to digital underwater photography.

Av (Aperture value) AE camera metering mode by which you choose the aperture and the metering system sets the shutter speed (also called aperture priority).

Backscatter Light reflected into the camera lens and showing up as snow on the finished photograph, resulting from suspended particles in the water.

Barrel distortion If you take a wide angle seascape with a wide angle or fisheye lens and notice that the horizon seems to curve, you are seeing barrel distortion. This is caused by the camera lens distorting an image so it appears spherised.

Beam angle The angle of a flashgun beam expressed in degrees.

Bokeh The blur or the aesthetic quality of the blur, in out-of-focus areas of a photographic image.

Bracketing (exposure) Taking several pictures of your subject at different aperture settings, flash distances or flash-power settings, with the objective of obtaining one perfect exposure.

Brightness range The range of brightness between shadow and highlight areas of an image. This is also referred to as 'dynamic range'.

'B' setting Brief or bulb. On this setting, the camera shutter stays open for as long as the release button remains depressed.

Buffer A buffer is RAM (Random Access Memory) inside a digital camera which can store images before they are written to the memory card. This means you can shoot a number of photographs without having to wait for each to be saved. Shoot too many and you will cause a delay before you can begin to re-shoot.

Calibration Calibration is altering the settings of a device so it conforms to a standard. You can calibrate the LCD of your camera's viewfinder, a screen monitor, scanner and printer so that what you see is accurate.

CCD (Charge-Coupled Device) The light sensor in a camera that records an image. It consists of millions of tiny light sensors, one for each pixel. The size of a CCD is measured in megapixels.

Centre-weighted metering An auto-exposure system that uses the centre portion of the frame to adjust the overall exposure value.

Close-up attachments Dioptres, teleconverters, wet lenses and other accessories which enable the camera to focus on subjects that are closer than the nearest distance that the lens would normally allow.

Colour temperature The measurement of colour expressed in Kelvin (K). The lower the Kelvin rating, the 'warmer' or more yellow the light. The higher the rating, the 'cooler' or more blue the light.

Compact flash A popular form of storage media for digital cameras.

Complementary colours A compositional tool for colour balance which indicates the colours that complement each other on the compositional colour wheel.

Composition The activity of positioning subjects within a frame or viewfinder. Photographers often aim to create a visual balance of all the elements within their photographs. They do this via careful composition.

Compression Refers to a process where digital files are made smaller to save on storage space or transmission time. Compression is available in two types: lossy, where parts of the original image are lost at the compression stage, and lossless, where the integrity of the file is maintained during the compression process. JPEG and GIF use lossy compression, whereas TIFF is a lossless format.

Contrast (composition) Photographers who position subjects with different characteristics in the frame together are said to be creating contrast in the composition. Placing a highly textured object against a smooth and even background creates a visual contrast between the two subjects and emphasises the main characteristics of each.

Contrast (exposure and tone) The difference (ratio) between the darkest and brightest parts. In a scene, this depends on lighting and the reflecting properties of objects. In a photograph, there is also the effect of exposure level, degree of development, printing paper, etc.

Crop factor The ratio of the focal length of a digital camera lens compared to a 35 mm film camera lens.

Cropping Cutting out unwanted (edge) parts of a picture in an imaging program such as Adobe Photoshop, or cropping at the printing or mounting stage.

Depth of field (D of F) The area that is in focus behind and in front of a subject. Depth of field is controlled by three factors: the focal length of the lens, the size of the aperture and the camera to subject distance. Diffuse lighting Scattered illumination, the visual result of which is gentle modelling of the subject with mild or non-existent shadows.

Diffuser A circular or rectangular disc placed over a flashgun to widen the beam of light. Diffusers also decreases light intensity from between one and three /-stops.

Digital image A visible image on a computer monitor formed by a stream of electronic data.

Digital zoom Many digital compacts can zoom in on an image by expanding it in-camera. The zoomed area looks bigger but contains the same number of pixels, so will look 'pixelated'. Digital zoom should not be confused with the superior, optical zoom.

Digitise This is the process by which analogue images or signals are sampled and changed into digital form.

Dioptre Magnification factor of a supplementary lens. The focal length of such a lens can be calculated by dividing a thousand by the power of the dioptre.

DNG (Digital Negative File) The digital camera file format from Adobe that was designed to provide a standard for RAW files HDR (High Dynamic Range).

Dome port A semi-spherical piece of glass or plastic used to eliminate the magnifying distortion caused by reflection.

dpi Dots per inch, a term used to indicate the resolution of a scanner or printer.

Dynamic range The measure of the range of brightness levels that can be recorded by a digital sensor.

Effective f-stop The actual /-stop value when you use a macro lens and focus for extreme close-ups.

Enhancement A term that refers to changes in brightness, colour and contrast that are designed to improve the overall look of a digital image.

EV (Exposure Value) This refers to the amount of shutter speed or aperture adjustment needed to double or halve the amount of light.

EXIF Stands for exchangeable image file. EXIF format enables image data, such as the date and time the shot was taken and exposure, to be stored onto the camera's memory card.

Exposure When you take a picture, the camera's light meter determines how long the shutter should be open for and how wide the aperture should be to gain the correct exposure. If a picture is too dark it's underexposed, and if it's too light then it's overexposed. Exposure compensation Adjusting the camera or metering system to give a greater or lesser exposure than that which the light meter considers to be correct. Most cameras now have an exposure compensation dial built in.

Exposure latitude (film latitude) The measure of a film's propensity to compensate for over- or underexposure. Slide film has small latitude. Plus or minus a half /-stop is common, therefore exposures have to be very accurate. Print film has much wider latitude.

Exposure mode Camera settings such as M (manual), A (aperture priority), etc. that determine which controls you have to adjust manually for an exposure and which ones the camera does automatically.

Fast lens — Slow lens — Lens speed Refers to the maximum aperture diameter, or minimum f number of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f number) is a fast lens because it delivers more light intensity to the sensor allowing a faster shutter speed to be used. A smaller maximum aperture (larger minimum f number) is 'slow' because it delivers less light intensity and reqires a slower shutter speed.

Fill-in flash This is additional light from a flashgun(s) to enhance colours and lighten shadows when natural light is the primary light source.

Firewire Faster than USB, this is a type of connection between computers and a range of different equipment, including digital cameras and card readers.

Fisheye lens A lens with a 180 degree field of view across the diagonal. Fisheye lenses offer maximum depth of field.

Fixed focus Non-adjustable camera lens set for a fixed subject distance.

Flare Scattered light that dilutes the image, lowering contrast and seeming to reduce sharpness. Mostly occurs when the subject is backlit or when using wide angle lenses with flash and extraneous light from the flash strikes the dome port.

Flash contacts Electrical contacts, normally within the mechanism of the camera shutter, which come together at the appropriate moment to trigger the flash unit.

Flash synchronisation The firing of a photographic flash coinciding with the shutter admitting light to image sensor. It is often shortened to flash sync.

Focal length This describes the magnifying power of a lens. The longer it is, the greater the magnification; conversely, the smaller it is, the wider the angle of the lens.

Focusing Changing the lens-to-subject distance to achieve a sharp image.

Focus priority (single servo) Auto focus mode by which you cannot release the shutter until the lens has sharply focused your subject.

Format Height and width dimensions of the picture area.

Fractional f-stop Any aperture setting which is between the full f numbers marked on the camera's aperture control ring. F19 is an example, being between f16 and f22. Frame One single image on a roll of film or digital sensor.

f-stops The various settings that control the camera lens aperture. The f-stop or f number indicates the relationship between the size of the aperture opening and the focal length of the lens, so a setting of f8 means that the diameter of the aperture is one-eighth of the lens focal length.

GIF An image file format designed for display of line art on the Web.

Greyscale A monochrome digital image containing tones ranging from white through a range of greys to black.

Highlights The brightest parts of a photo.

Histogram A graphic representation of the range of tones from dark to light in a photo. Some digital cameras include a histogram feature that enables a precise check on the exposure of the photo.

Hot shoe Mounting on top of the camera to which the flash is attached.

Image browser An application that enables you to view digital photos. Many browsers also allow you to rename files, convert photos from one file format to another, add text descriptions, and more.

Image stabilisation This helps you in taking handheld shots that would otherwise require a tripod by compensating for small and fast movements (shaking). It is especially useful on long lenses in less than ideal lighting.

Infinity A distance so great that light from a given point reaches the camera as virtually parallel rays — in practice, distances of about 1000 times the focal length or more. It is written on lens focusing mounts as 'inf' or the symbol 8.

Interchangeable lens A lens that can be removed from a camera to be replaced by another lens.

Interpolation A mathematical method of creating missing data. An image can be increased from 100 pixels to 200 pixels through interpolation. There are many methods of interpolation, but one simple method is to generate a new pixel by using the average of the value of the two pixels on either side of the one to be created.

Inverse square law The physical law that causes light from a flash to fall off in such a way that as flash-to-subject distance doubles, the light falls off by a factor of four. It forms the basis of flash guide numbers and close-up exposure increases.

ISO (International Standards Organisation) In the ISO film speed system, halving or doubling of speed is denoted by halving or doubling the ISO number. It also incorporates the DIN figure, e.g. ISO 400/27 film is twice as sensitive as ISO 200/24.

JPEG A file format, designed by the Joint Photographic Experts Group, which has inbuilt lossy compression that enables a massive reduction in file sizes for digital images.

Kelvin Measurement unit of lighting and colour temperature.

Kilobyte 1024 bytes.

Landscape An image taken with the camera in its normal horizontal orientation.

Latitude See Exposure latitude.

LCD monitor A small LCD liquid crystal display screen on the back of the camera, used to compose or look at photographs.

LED (Light Emitting Diode) Lights and symbols displayed in the camera viewfinder to give exposure data.

Lens Filter A piece of transparent glass, plastic or gelatine placed over the camera lens to adjust colour in natural light photography or to balance the light when flash is used.

Lens speed The widest aperture to which a lens can be opened. The wider the maximum aperture of a lens, the faster it is said to be.

L-ion Lithium-ion is a popular type of rechargeable battery. It holds more power and does not suffer from the 'memory effect', where a battery when recharged only registers the additional charge and not its full capacity.

Line (composition) 'Line' is one of the strongest visual elements that photographers can use to help compose their pictures. Often, line is used to direct the attention of the viewer towards a certain part of the frame or a specific focal point.

Macro lens Intended for close-up photography, able to focus on subjects at close distances. The majority of macro lenses now provide life-size magnification (1:1).

Matrix metering An exposure system that breaks the scene up into a grid and evaluates each section to determine the exposure.

Megapixel One million pixels. Used to describe the resolution of digital camera sensors.

Modelling light (aiming light) A small light, often a torch, which is attached to a flashgun and shows the direction in which the flash is pointing and also the effect that the angle of the flash will have on the subject.

Monochrome Single coloured. Usually implies a black and white image, but also applies to one which is toned (e.g. sepia).

Mood The mood of a photograph refers to the emotional content of the picture.

M setting Indicates the cameras is in Manual exposure mode.

Negative space Everything in the photograph that is not the subject.

Neutral density filter A filter that reduces the amount of light entering the camera lens.

Ni MH Nickel metal hydride battery. Rechargeable, ecologically safe and very efficient.

Noise Misinterpreted pixels found in a digital image, usually occurring in longer exposures, which can be seen as misplaced or random bright pixels in the picture.

Normal lens Lens that has an angle of view similar to that of the human eye. Underwater, that length is considered to be 35 mm (on film).

Open up To increase the size of the lens aperture. The opposite of stop down.

Optical zoom Digital compacts have optical zoom lenses. This means that they can be adjusted to magnify the scene before you zoom in or, alternatively, zoom out to capture a wide angle scene.

Orientation sensor A sensor that knows when you turn the digital camera to take a vertical shot and rotates the picture so it won't be displayed on its side when you view it.

Overexposure An image that appears too pale because of too much light reaching the sensor.

Panning Rotating or swinging the camera about a vertical axis to follow the movement of a subject. Carried out correctly with the shutter open, this should produce a sharp subject against a blurred background. Plenty of practice is required to master this technique.

Parallax error Viewpoint difference between the picture seen in the viewfinder and as seen by the camera lens.

Pattern (composition) Repeating subjects that have similar characteristics such as colour, shape and texture create a strong visual element that is often referred to as pattern. Pattern can be used in a similar way to line and colour as a way to balance compositions and direct the viewer's eye throughout the frame.

Pixel Short for 'picture element', and refers to the smallest image part of a digital photograph.

Pixelisation An effect seen when you enlarge a digital image too much and the pixels become obvious.

Polariser Grey-looking polarising filter, able to darken blue sky when at right angles to the sunlight, and suppress reflections from (non-metallic) surfaces.

Port Glass (or Perspex) window that is attached to the camera housing, through which the lens 'looks' underwater. A flat port is associated with macro lenses, and a dome port with wide angle lenses.

P (Program) mode Indicates that the camera is in the Program exposure mode, in which the camera selects both aperture and shutter speed automatically.

Prosumer A broad term that refers to a digital compact camera with a range of manual controls, many of which can be found on an SLR. They are capable of taking pictures to the very highest standard, and usually have a minimum resolution of 5 megapixels.

Rear curtain synchronisation Here, the flash fires an instant before the second (rear) curtain of the focal plane shutter begins to move. When slow shutter speeds are used, this feature can create a blur effect from the ambient light — e.g. flowing light patterns following a moving subject with subject movement frozen at the end of the light flow.

Reciprocity law failure The effect of dim light, or small lens aperture, can be counteracted by giving a long exposure time, but this reciprocal relationship (half the brightness, double the exposure time) increasingly breaks down with exposure times beyond 1 s. The film then behaves as if having a lower speed rating. Colour films may also show incorrect balance.

Recycle time The time it takes for a flashgun to recharge between flashes.

'Red eye' The iris of each eye in portraits shows red instead of black; this is caused by using flash directed from close to the lens.

Refraction Change of the direction of a ray of light passing obliquely from one transparent medium into another of different density, e.g. from air into water. Underwater, this causes objects to appear closer and larger than they actually are.

Relative f-stops /-stops marked on the camera's aperture ring, as opposed to effective f-stops. Reproduction ratio The size of the image on film or sensor compared with its actual 'real-life' size.

Resolution An indication of the sharpness of images on a printout or the display screen. It is based on the number and density of the pixels used — the more pixels used in an image, the more detail can be seen and the higher the image's resolution.

RGB The colour system used in most digital cameras, where red, green and blue light are captured separately and then combined to create a full colour image. Ring-flash A ring-flash does just what its name implies, encircling the camera lens with a flash tube so that the light is projected forward from the camera.

Scanner An input device that uses light to read printed information, including text, graphics and bar codes, and transfers it into the computer in a digital format.

Short focal-length lens (wide angle) A lens that provides a wide angle of view of a scene, including more of the subject area than does a lens of normal focal length.

Shutter The device in the camera that opens and closes to let light from the scene strike the image sensor and expose the image.

Shutter lag The time between pressing the shutter release button and the camera actually taking the shot. This delay varies quite a bit between camera models, and used to be the biggest drawback of digital photography. The latest digital cameras, especially the prosumer and professional SLRs, have virtually no lag time.

S (Shutter priority) mode An automatic exposure system in which you set the shutter speed and the camera selects the aperture (/-stop) for the correct exposure.

Shutter speed The length of time for which the shutter is open and light strikes the image sensor.

Single lens reflex (SLR) A type of camera with a lens that is used both for viewing and for taking the picture. Slave flash A flash that is activated by the light from another flashgun. The 'slave' can be turned on or off at the touch of a switch.

Smart media A popular form of flash memory card.

Snell's window The circular arc visible on the under-surface of the water, caused by the effect of refraction.

Spot metering A metering method based on a small circle in the centre of the viewfinder to calculate the best possible exposure.

Stepless shutter speeds Infinite number of shutter speeds available on modern cameras.

Stop An aperture setting that indicates the size of the lens opening.

Stop down To decrease the size of the lens aperture. The opposite of open up.

Teleconverter A device used to increase the effective focal length of a lens that consists of optical glass. It is mounted between the camera and the lens, and usually comes in two different sizes: 1.4x and 2.0x. A 1.4x tele-converter increases the focal length by 1.4 times, while a 2.0x increases focal length by 2.0 times.

Telephoto lens A lens that provides a narrow angle of view of a scene, including less of a scene than would a lens of normal focal length, and therefore magnifying objects in the image.

Through-the-lens (TTL) metering Measuring exposure by a meter built into the camera body, which measures the intensity of light passing through the picture-taking lens.

Thumbnail A low-resolution preview version of larger digital image files used to check before opening the full version.

TIFF A popular lossless image format used in digital photography.

Time exposure General term for a long-duration exposure.

Tone (subject matter) Tone can also refer to the mood of a picture. When the tone of a photograph is said to be 'dark', then the subject matter and/or the way that the content is depicted can be emotional, complex, sometimes sad, confrontational and generally thought-provoking.

Translucent Transmitting but at the same time also diffusing light (as with tracing paper).

Underexposure Exposing the film or sensor to less light than is needed to render the scene as the eye sees it. This results in too dark a photograph.

Up rating Shooting film at more than the manufacturer's suggested speed rating (e.g. exposing 400 ISO film as if 800 ISO).

Viewpoint The position from which camera and photographer view the subject.

Vignetting Progressively diminished illumination on the film from the centre to the corners. There are two kinds of vignetting: natural vignetting caused by the lens, and vignetting that is caused by improper use of accessories such as a lens hood or filter.

VR (Vibration Reduction) The name which Nikon have given to there image stabilisation mechanism.

White balance An automatic or manual control that adjusts the brightest part of the scene so it looks white. Cameras have pre-set options, such as sunny, cloudy, flash, etc.

Wide angle lens See Short focal-length lens.

Zoom lens A lens that offers several lenses in one by allowing the focal length to be altered at will. The minimum and maximum focal lengths available are made clear by the way in which zoom lenses are described and labelled (e.g. 17 mm—35 mm).

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