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Sensors, Pixels and Image Sizes

Image Sensors and Pixels

Click to see how pixels are printed using dots of colored ink.
As you saw earlier, digital photographs are actually mosaics of millions of tiny squares called picture elements—or just pixels. Like the impressionist painters who painted wonderful scenes with small dabs of paint, your computer and printer use these tiny pixels to display or print photographs. To do so, the computer divides the screen or printed page into a grid of pixels. It then uses the values stored in the digital photograph to specify the brightness and color of each pixel in this grid—a form of painting by number.

This reproduction of the famous painting "The Spirit of '76" is done in jelly beans. Think of each jelly bean as a pixel and it's easy to see how dots or pixels can form images. Jelly Bean Spirit of '76 courtesy of Herman Goelitz Candy Company, Inc. Makers of Jelly Belly jelly beans.

Now that you understand a little about pixels and images, let's introduce one surprising fact: A pixel has no size or shape. At the time it's born, it's simply an electrical charge on a photosite much like the static electricity that builds up on your body as you shuffle across a carpet on a dry day. A pixel is only given size and shape by the device you use to display or print it. Understanding how pixels and image sizes relate to one another takes a little effort but you need to bring nothing more to the process than your curiosity and elementary school arithmetic skills.

A pixel begins its life on the camera's image sensor during that flickering moment when the shutter is open. The size of each photosite on the image sensor can be measured, but the pixels themselves are just photons, soon to be converted into electrical charges, and then into zeros and ones. These numbers, just like any other numbers that run through your head, have no physical size.

Although the captured pixels have no physical dimensions, a sensor’s size is specified just like a digital photo’s, except the count is the number of photosites that it has on its surface instead of pixels. In most cases the numbers of photosites and the number of pixels are roughly the same since each photosite captures one pixel. (see Tip box for more on this).

Since pixels stored in an image file have no physical size or shape, it's not surprising that the number of pixels doesn't by itself indicate a captured image's sharpness or size. This is because the size of each captured pixel, and the image of which it's a part, is determined by the output device. The device can spread the available pixels over a small or large area on the screen or printout.

If the pixels in an image are squeezed into a smaller area, the image gets smaller and the perceived sharpness increases (from the same viewing distance). Images on high-resolution screens and printouts look sharper only because the available pixels are smaller and grouped into a small area-not because there are more pixels.

As pixels are enlarged, an image is spread over a larger area, and its perceived sharpness falls (from the same viewing distance). When enlarged past a certain point, the individual pixels begin to show— the image becomes pixilated.

To visualize this concept, imagine two tile mosaics, one with small tiles and one with large.

  • If both mosaics cover an area of the same size, the one created using small tiles has more tiles so it has sharper curves and more detail.
  • If there are the same number of large and small tiles, the area covered by the small tiles is smaller. When viewing both mosaics from the same distance, the smaller one looks sharper. However, if you view the small mosaic from close up, its sharpness and detail appear almost identical to the larger one viewed from farther away.
To make an image larger or smaller for a given output device, you must add or subtract pixels. This process, called resampling, can be done with a photo-editing program, or by an application you're using to print an image.
  • When an image is resampled to make it larger, extra pixels are added and the color of each new pixel is determined by the colors of its neighbours.
  • When an image is resampled to make it smaller, some pixels are deleted.

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