Everything you need to know about digital photography


Photography has its fair share of technical terms and buzz-words. In this glossary section, some of those terms are defined.

Aperture - the circular or near-circular opening inside a lens which controls how much light is allowed to reach the sensor or film.

Aperture Priority - an automatic exposure mode in which the photographer selects the aperture/F-number to use, leaving the camera to adjust the shutter speed according to the light level, so as to make a well-exposed photograph. Frequently abbreviated to A or Av on a camera's mode selector.

Aspect Ratio - the relationship between the width and height of an image. So a 4000x3000 photo would have an aspect ratio of 4:3

Bridge Camera - a digital camera with non-interchangeable lens, but having a similar body shape to a DSLR although generally smaller.

CCD (Charge Coupled Device) - a sensor technology that converts light levels into electrical charge and then allows those charge values to be sequentially read out and converted to digital data.

CMOS - a technology used for fabricating electronic chips. Sensors built using this technology are less expensive than CCD sensors, and can contain associated image processing circuitry on the same chip.

Compact Camera - a camera typically with a relatively thin rectangular shape, a non-interchangeable lens, an LCD display functioning as the principal viewfinder with perhaps an optical viewfinder that does not share the main imaging lens.

Crop Factor - in a DSLR, relates the sensor size to the size of a full-frame sensor. A crop factor of 1.5 indicates a sensor with 2/3 the dimensions of a full frame. A particular lens on such a camera would effectively magnify 1.5x as much as the same lens on a full frame DSLR.

Cropping - selecting an area of a photograph which contains the desired image, discarding the outlying portions.

Depth of Field, DOF - the range of distances either side of the true focal distance, for which objects are still in sharp focus. Depth of field increases is the aperture is reduced, or (equivalently) the f-number is increased.

Diaphragm, Iris - a mechanical device containing multiple overlapping blades arranged in a circle, and pivoted so that they can all be simultaneously swung towards or away from the centre of the circle - thereby controlling the amount of light allowed through the central opening. The blades are generally curved so that the opening approximates a circle, since this shape creates fewest visual artefacts in a photograph. Odd number of blades are usually preferred for the same reason.

Digital Zoom - image enlargement achieved by selecting a portion of the image from the centre of the sensor, and scaling it up digitally. Generally featured on budget cameras, and does not appreciably increase image resolution, although it may avoid the need to crop at a later stage.

DSLR - Digital Single Lens Reflex camera. A digital camera in which the optical viewfinder 'looks out' through the same lens that is used for imaging, using a mirror which is moved out of the way when the shutter is opened to take a photograph. The lens is generally interchangeable.

Electronic Viewfinder - an image displayed on a small LCD display inside the camera, viewed by the photographer through an eyepiece lens in order to compose the photograph. Typically the source of the image is the main sensor, so it is possible to frame the final image accurately. Depending on the resolution of the LCD display, it may be difficult to judge detail visually. There may also be some delay in the viewfinder sytem.

EVIL - Electronic Viewfinder, Interchangeable Lens - a variant of SLR which avoids some optical complexity by replacing the optical viewfinder with an electronic version. The camera body can thus be considerably more compact, whilst retaining many advantages of a DSLR including the ability to swap lenses.

Exposure - the amount of time that the camera shutter is kept open when taking a photograph. The longer the exposure, the brighter the image will appear. 'Exposure' is also sometimes used to refer to the image itself.

F-Number, F-Stop - effectively the ratio between the focal length of a lens and the size of the aperture. Normally specified in numbers that sequentially increase by a factor of approximately 1.4, with each step halving the amount of light transmitted and hence doubling the required exposure time. Higher F-numbers produce a greater depth of field.

Flare - bright areas in a photograph caused by reflections from lens surfaces or scattering from imperfections. Reflections may take the form of circles or polygons, shaped by the camera aperture, and coloured by the anti-reflective coating on the lenses. Scattered light will generally appear as a diffuse brightening. You can reduce flare by using a lens hood to shield any bright lights which are just outside the field of view of your photograph.

Focal Length - the distance between a lens and the sensor when the lens is focused at infinity (i.e. on the horizon). A lens with a long focal length is referred to as a telephoto lens. Short focal lenght lenses are called wide-angle. Lenses with variable focal length are zoom lenses.

Iris - see Diaphragm

ISO - a measure of sensitivity to light - or how much electronic amplification is applied to the signal coming from the sensor before it is converted to a digital quantity. A typical 'base level' is 100 or 200. Higher settings boost the sensitivity but also amplify any noise, leading to image graininess.

Optical Viewfinder - a system of lenses which the photographer can look through to compose a photograph. May include the camera's main imaging lens in an SLR design, or small lenses parallel to the imaging lens in a typical compact camera.

Optical Zoom - a change in image magnification achieved by adjusting the camera's lens. Produces full-resolution images at all zoom settings.

RAW - an image format which preserves all the data captured by the sensor, as well as the camera settings at the time. This allows software on a PC to recover image information (such as highlight and shadow detail) that might have been lost if the camera saved in a format such as JPEG, which is not able to store such detailed information, and which loses data due to the compression algorithm used.

Sensor - the electronic device in a digital camera that takes the place of a photographic film, converting a light image into electronic data.

Shutter - the mechanical device that controls when, and for how long, light is allowed to reach the sensor of a camera. Normally consists of two 'curtains' that move in front of the sensor - the first curtain slides down to start the exposure, and the second follows it shortly afterwards to end the exposure. For short shutter times, the effect is of a slit moving across the sensor, ensuring all areas receive an equal total exposure duration even if not all at exactly the same time.

Shutter Priority - an automatic exposure mode in which the photographer selects the shutter speed to use, leaving the camera to adjust the aperture according to the light level, so as to make a well-exposed photograph. Frequently abbreviated to S or Tv on a camera's mode selector.

Stabilisation - a system designed to reduce the blurring effect of camera shake. This can be achieved by moving elements within the lens to compensate for camera movement, by moving the sensor itself, or by selecting the optimum time to actually release the shutter after the button is pressed.

Viewfinder - see Optical Viewfinder, Electronic Viewfinder

Zoom - see Optical Zoom, Digital Zoom