While most photographers focus on the number of megapixels to gauge a camera's resolution, the actual resolution is more complex than the number of pixels on an image sensor. For example, the camera may not use all the pixels in the digital image; some pixels may be used for other imaging tasks. Other factors come into play as well, such as sensor size and digital noise, which is the grainy quality of a low-light digital image.

Sensor size must be a consideration in evaluating megapixel count. Many point-and-shoot digital cameras boast an excess of 10 megapixels, but these 10 million pixels are crowded onto a sensor less than half the size of your fingernail! The 10 million pixel sensor on a Digital Rebel XTi is much larger physically than a 10 megapixel sensor on most, if not all, point-and-shoot cameras boasting the same megapixel resolution. Larger photo sensors can gather more light, have less digital noise, and have better dynamic range. In this case, bigger is better.

Each pixel in a camera sensor contains one or more light-sensitive receptors that convert the incoming light into an electrical signal. This signal is then processed into the colors used in the final image, and the brightness of individual pixels is amplified or decreased. A common cause of digital noise is overamplification of the signal coming from the photosite. If the same pixel is exposed several times by the same amount of light, the resulting color values are not ...

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