Resolution = Information Digital Imaging

     Resolution is one of the most important concepts to understand in digital imaging and especially in digital photography. The term resolution describes both pixel count and pixel density, and in a variety of circumstances these concepts are used interchangeably, which can add to misunderstanding. Camera resolution is measured in megapixels (meaning millions of pixels); both image file resolution and monitor resolution are measured in either pixels per inch (ppi) or pixel dimensions (such as 1024 by 768 pixels); and printer resolution is measured in dots per inch (dpi) . In each of these circumstances, different numbers are used to describe the same image, making it challenging to translate from one system of measurement to another.
   This in turn can make it difficult to understand how the numbers relate to real-world factors such as the image detail and quality or file size and print size.  The bottom line is that resolution equals information. The higher the resolution, the more image information you have.
   If we’re talking about resolution in terms of total pixel count, such as the number of megapixels captured by a digital camera, we are referring to the total amount of information the camera sensor can capture, with the caveat that more isn’t automatically better. If we’re talking about the density of pixels, such as the number of dots per inch for a print, we’re talking about the number of pixels in a given area. The more pixels you have in your image, the larger that image can be reproduced. The higher the density of the pixels in the image, the greater the likelihood that the image will exhibit more detail or quality.

Megapixels vs. Effective Megapixels
        Digital cameras are identified based on their resolution, which is measured in megapixels. This term is simply a measure of how many millions of pixels the camera’s image sensor captures to produce the digital image. The more megapixels a camera captures, the more information it gathers. That translates into larger possible image output sizes. However, not all the pixels in an image sensor are used to capture an image. Pixels around the edge are often masked, or covered up.
     This is done for a variety of reasons, from modifying the aspect ratio of the final image to measuring a black point (where the camera reads the value of a pixel when no light reaches it) during exposure for use in processing the final image. Because all pixels in the sensor aren’t necessarily used to produce the final image, the specifications for a given camera generally include the number of effective megapixels. This indicates the total number of pixels actually used to record the image rather than the total available on the image sensor.