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The Mother Of All Explanations

If there's one mystifying topic that's endemic to digital photography, it's resolution. Not because it's difficult to understand; it's really quite simple. It just seems confusing because almost everyone who tries to explain it, ends up using the wrong terminology which only makes things worse. Hopefully, this will be the mother of all explanations and will cause the light bulb to finally flash on.

Camera Resolution­ Measured In Pixels

Unlike conventional cameras that require film to record pictures, a digicam's imaging surface is made up of rows of tiny sensors that capture color and light information which is then electrically converted into digital data­ pixels (from the words picture elements) that form a photograph. The total sensor array of most digicams is called a charge-coupled device (CCD).

The more sensors that can be packed onto the CCD's surface, the higher the resolution will be­ resulting in a final image with a high pixel count and finer detail. In rough terms, think of low and high resolution as gravel compared to sand; one's coarse, the other's fine. A digicam that captures an image consisting of 640 horizontal by 480 vertical pixels, is said to have a resolution of 640 by 480 pixels or 307,200 pixels (arrived at by multiplying the two dimensions). Once you reach a million pixels, you're in the MegaPixel (MP) range.

Digicam resolution is always expressed in pixels (not pixels-per-inch) because CCDs can be different sizes. You can have a 1/2-inch CCD with a million pixels on it and a 1/3-inch CCD with the same number. If you did the math, each would have a different number of pixels-per-inch though they'd have exactly the same resolution. So measuring camera resolution using pixels-per-inch is a "no no." Measuring printed image resolution, on the other hand, is a whole different ballgame.

Image Resolution– Measured In Pixels-Per-Inch (ppi)

Unlike camera resolution, which counts pixels on the CCD and is an unchanging number, image resolution is measured in pixels-per-inch (or ppi) and changes with the size of your photo. For example, imagine an image from a digicam that has a camera resolution of 1.8 megapixels printed at 4 x 5 inches on a piece of rubber. If you stretch the rubber to get an 8 x 10 inch picture each of those 1.8 million pixels becomes bigger. If you keep increasing the size of the photo the pixel size will continue to increase until you can see each separate pixel, resulting in an image that looks jagged and blocky.

To extend the example, let's arbitrarily assume that at 4 x 5 inches, 300 pixels were lined up along each inch of a photographic print, resulting in 1200 pixels on the 4-inch side and 1500 pixels on the 5-inch side. But when you stretched the image to 8 x 10 inches, the pixels got bigger so that only 150 of them could fit along an inch of space; nevertheless there were still the same 1200 pixels on the 8-inch side and 1500 pixels on the 10-inch side.

So while the number of camera resolution pixels stayed the same (1200 x 1500 = 1.8megapixels ), the number of image resolution pixels per inch changed with the picture size. Here's the bottom line: smaller-sized prints of the same photo will have smaller-sized pixels and more of them to the inch (a higher ppi count); larger-sized prints will have larger-sized pixels and less of them to the inch (lower ppi). And that's all there is to image resolution. Now on to printer resolution.

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