A digital photographic image is made up of a mosaic of individual cells, called Pixels - reputedly from "Picture Element". Each pixel is uniformly a single colour; the number of different colours available (the colour depth) dependent on the bit width of the storage format or image sensor (such as a CCD).
The maximum possible sharpness of an image is largely determined by the number of pixels used to store, display and/or print it; more pixels means smaller pixels, which can capture increased detail. Similarly, a wider bit width allows more colour differences to be rendered, and so better subtlety of tone. The number of pixels (or, more properly, the number and bit-width/colour-depth) is often called the resolution of the image. More pixels (and greater bit widths) can mean greater possible quality, but also raised memory or disk storage requirements, longer processing times and slower transmission over networks. Too few pixels (or too shallow colour depth) can result in the pixels becoming noticeable in an image, which can show "jaggies" - jagged edges to lines and loss of fine detail.
A Megapixel is a million pixels - occasionally abbreviated to Mpx or Mp. Typical digital cameras have resolutions specified in megapixels.
The "native" resolution of a camera is determined by the resolution - or number of pixels - of the image sensor, although some cameras use software to interpolate their sensor images into higher resolution pictures.
Pixels in an image are usually square (i.e. the same width as height), and arranged in a rectangular grid. However, there are exceptions; some computer displays (e.g. the EGA "standard") have had rectangular pixels (e.g. taller than they are wide), and some image sensors have their pixels in non-rectangular grids (e.g. the octagonal pixels of the Fuji Super CCD). These exceptions are reasonably rare - and for most purposes are usually processed into square pixels once they leave the camera.
How many megapixels are useful?
|The era of 12 MP system cameras (2007-2011) in one picture:|
Panasonic Lumix DMC-G1, Sony Alpha DSLR-A700, Olympus Pen E-PM1
image by Uwe Kulick (Image rights)
Around the year 2006 there was a discussion whether more and more megapixels in the newest cameras were useful anyway or not, especially in Germany. The thesis "6 megapixels are enough" was spread, even by experts like the renowned German camera tester Uschold, and seemed to be proven by highly recommended established top consumer & pro cameras like Minolta DiMAGE A1 (5 MP), Olympus E-1 (pro camera, 5.5 MP), Fujifilm FinePix F30 (6 MP), Konica Minolta Alpha Sweet Digital (6 MP) and Nikon D70(6 MP) which all performed prettily well, whilst Sony's first still photo system camera of 2006 Alpha 100 offered 10 MP, but was much weaker than 6 MP DSLRs concerning light sensitivity. Thus Sony's A100 was sometimes called Sony ISO 100 by some critics - a fine-weather camera and maybe good with flash or long ISO 100 exposure in darker surroundings. Smaller pixels, less image quality, that was well according to the too-much-megapixel-critics' theory. Just one year later the same camera maker Sony could make its customers happy with its first CMOS-sensor-based semi-pro DSLR Alpha 700 of 2007. 12 megapixels, double as much pixels as the so well established 6 MP cameras offered. The 6-megapixel thesis was ready for the waste paper basket!
The cameras mentioned above all have an APS-C size sensor or a smaller one. People who switched early to full-frame sensor system cameras might have had earlier the joy of 12 megapixels or even more in nice quality even at higher ISO settings. But sometimes camera makers came back to the "less pixels are better" theory, for example the 19MP super-zoom digicam Panasonic Lumix TZ61 was paired by a twin (TZ71) with the same sensor size but just 12 megapixels for the achievement of more light sensitivity. And Sony found the way back to the 12 megapixels even on full-frame sensor for the extremely light-sensitive A7s system camera series of its Alpha system. 12 megapixel images are already sufficient for reusage in print media. Other newer digital full-frame cameras also offer pretty light sensitivity for low light situations despite of much more megapixels, but the mentioned low light specialist cameras seemed to have found their users since they're already available in third generation. And the answer how many megapixels you need lies not just in the light-sensitivity question, but also in the strength of the computer that You use for image improvement, in the strength of the lenses for Your system camera, in the love for detail that You experience in photography, in the money you want to invest in memory cards, etc.