Digital cameras capture light with image sensors. A pixel is an individual image sensor. The word pixel is short for picture element. One million pixels are called a megapixel. An effective megapixel is a measure of how many pixels actually record a printable image in a given camera or recording device.
The first professional digital cameras were introduced in the late 1990s. They had about 3 megapixels and cost around $5,000. As new brands came on the market, the performance standard became the number of megapixels a camera could capture. At first this was a good benchmark for the quality of the output. Intense competition in the consumer electronics business drove camera manufacturers to claim the most megapixels as their advantage. This is when the distinction between effective and total megapixels became important.
A photo comprises the image that is printed plus a few pixels around the edge that are used to establish a black point. They are used in the processing of the image but do not appear when the image is printed. These pixels represent the difference between total and effective pixels.
While effective megapixels were one of the most important technical advantages for a long time, the importance of the megapixel has diminished as camera and now phone offerings have improved. For example, as of 2011 digital single-lens reflex (DSLR) cameras are sold with up to 36 megapixels and even phones are now regularly sold with 5-megapixel cameras. A 5 megapixel image can be printed as large as 9 inches by 7 inches and still retain a high-quality 300 dpi. So megapixels become a consideration only if the image will be printed or viewed in a larger format.
One area where effective megapixels might remain a consideration is with the increased use of powerful digital zoom lenses. An optical zoom lens focuses on a distant object but retains the full use of the camera's image sensors and therefore its full resolution. A digital zoom lens focuses on the center portion of the image and magnifies it, giving the illusion of an optical zoom lens but without the higher resolution. So a digital zoom lens may be said to use fewer effective megapixels than an optical zoom lens on the same camera.
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