The pixel-count (e.g Canon 20D: 8.5 megapixel) is not the exact pixel count. In this case, the Canon 20D outputs 3504 x 2336 pixels, which is equivalent to 8.2MP (those are the effective pixels.). The amount of the pixels “mounted” on the sensor are more than the amount of pixels used for the images.

Many manufacturers prefer using the “unreal”-pixel-count (full pixel count) for marketing purposes.

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