Line pairs are used to measure resolution. A line pair is a pair of adjacent dark and light lines. A resolution of 10 lines per millimeter is equivalent to 5 dark lines alternating with 5 light lines (or 5 line pairs per millimeter). For example, a 640 x 480 image would have 307,200 pixels, or approximately 3.1 megapixels; a 3872 x 2592 image would have a10,036,224 pixels, or approximately 10 megapixels. The more pixels an image has, the higher the resolution.

Below is an illustration of how the same image might appear at different pixel resolutions.

Image at different pixel resolutions

The number of megapixels is not an absolute measure, but only indicates the potential image resolution. Important factors that affect resolution include sensor design, the lens quality, focus (distance and length), aperture, position in the image field, orientation (horizontal, vertical, and diagonal), scene contrast, and vibration.

There is a wide misconception about the resolving power of digicams. For instance, my 12-megapixel digital camera does not have twice the resolving power of a 6-megapixel camera. It only has twice that of a 3-megapixel camera. Why? Because images are two-dimensional objects. When we double the resolving power, there is a fourfold increase in pixels (of equal quality) and not just a twofold increase!

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