Effective pixels

Camera makers quote two megapixel figures. The bigger, ‘gross’ figure counts all the photosites on the sensor, but many of those around the edges are used for calibration and other technical purposes, so makers also quote the ‘effective’ pixels, which are the ones actually used to make the image. This is the important figure.

Demosaic

Process where the camera (or RAW conversion software) takes the ‘mosaic’ of red, green and blue pixel data from the sensor and converts it into full-colour information.

CMOS

This is the most common type of sensor in today’s digital cameras. One of its main advantages is its lower heat output compared to the CCD sensors used in the past. This makes it particularly suitable for cameras with larger sensors and mirrorless cameras where the sensor is always ‘on’.

CCD

An older type of digital camera sensor still used on a few specialised cameras but now mostly replaced with more efficient CMOS sensors. These produce less heat and noise and are better suited to use in cameras with full time live view and video features.

Back-illuminated sensor

A newer type of sensor where the circuitry has been moved to the back so that the light receptors on the front are unobstructed. This gives a modest but useful improvement in light-gathering power, digital noise and overall image quality, but it’s not the dramatic technical leap that manufacturers often suggest.

Bayer sensor

Most camera sensors use a single layer of photosites (pixels). These are only sensitive to light, not colour, so a mosaic of red, green and blue filters (the ‘bayer pattern’) is placed on top of the sensor’s photosites so that individually they capture red, green or blue light. When the camera processes the sensor data to produce an image, it ‘demosaics’ the red, green and blue data, using colour information from surrounding photosites to ‘interpolate’ full colour data for each pixel.

Aspect ratio

This the picture’s proportions as width versus height. DSLR sensors have a 3:2 ratio, so that photographs are 3 units wide to 2 units high. Most compact camera sensors have a slightly squarer 4:3 aspect ratio. It doesn’t matter what the units are – the ratio stays the same, so a photo could measure 3 inches by 2 inches or 6 meters by 4 meters and still have the same 3:2 aspect ratio. You can shoot in different aspect ratios by cropping the sensor area. HD video is shot in a wider 16:9 ratio.

Anti-aliasing filter

Another name for the ‘low pass’ filter fitted in front of most camera sensors. It’s designed to prevent digital artefacts such as moiré patterns and colour fringing caused by interaction between fine linear or rectangular patterns in real-world subjects and the camera’s rectangular grid of photosites.

APS-H sensor

This is a relatively uncommon sensor size mid-way between APS-C and full frame. Canon used it for its EOS-1D high-speed pro sports/press photography DSLRs before these were merged with the introduction of the full frame EOS-1D X. Canon has since announced the development of a 250MP APS-H format sensor, though this has not yet been used in any commercial product. Sigma, meanwhile, has announced a new Sigma SD Quattro H mirrorless camera with a new APS-H format Foveon sensor. APS-H sensors measure approximately 30 x 20mm, or a couple of millimetres less.

APS-C sensor

This is the most common sensor size in cameras designed for enthusiasts and experts and it’s found in consumer DSLRs, mirrorless compact system cameras and some high-end compacts. APS-C sensors are around half the size of a full-frame sensor or the 35mm negative, and measure approximately 24 x 16mm. They have a crop factor of 1.5x, which means that you have to multiply the lens’s focal length by 1.5x to get its effective focal length in 35mm/full frame camera terms.

1.5-inch sensor

A unique sensor size used by Canon in its top PowerShot compact camera, the G1 X II. It’s just a little smaller than the APS-C format used by most DSLRs and larger than the 1-inch sensors used in other high-end PowerShot models, so it gets close to the quality of a good interchangeable lens camera.