Image sensors - Effective & image pixels

Digital camera manufacturers often quote the total number of pixels of the sensor to indicate the camera's resolution. However, not all of these pixels are used for recording the image.

For a digital camera to know what black is, it needs to take a dark current reading, which is the base value of a completely black pixel. For this purpose some of the vertical columns and horizontal rows on the edge of the sensor are covered with a black dye – so called Video Signal Shading – and cannot be used for recording the image. Within this number of “black” pixels the manufacturer has the flexibility to choose the output image size which will fit into a standard aspect ratio such as 4:3 (1600 x 1200 or 2048 x 1536 pixels) or 3:2 (the equivalent of 35 mm film) and crop the CCD's pixels accordingly.

When comparing digital cameras and their resolutions, it pays to be aware of the effective number of pixels, as often the total number of CCD pixels is quoted as an advertising ploy. (compare flatbed scanners and their interpolated resolutions quoted in advertisements as opposed to their effective resolutions). The effective pixel count is the number that matters, because this is the actual number of pixels that can be used for recording an image.