The representational power of digital photography begins with individual picture elements called "pixels." If you enlarge any digital image enough you will see that the apparently continuous tonalities of the image are actually created from these tiny individual image tiles.
And yet, despite how familiar we are with the general idea of pixels, there is still a lot of confusion about what they actually are.
There are three essential things you need to understand about pixels and each of these has important implications for the quality of digital photographs:
1. There are two general types of pixels: fixed-sized Hardware Pixels and variable-sized Image Pixels.
2. Although image pixels can be almost any size, in black-and-white digital images each pixel can only be one individual tone of black, gray or white. In color digital images each pixel can only be one single color.
3. Although each individual pixel in an image can only be one tone or color, the number of different tones or colors a pixel is allowed to be by current computers can be vast!
Hardware pixels are physical things built into digital sensor chips, computer monitors, scanners, and ink jet printers.
The function of hardware pixels is to gather, display, and reproduce the visual information that we see as Image Pixels.
For example, on the surface of a computer monitor, a "pixel" is a fixed and rigid physical entity that can never change size. The resolution of monitors is set to values like 72 or 96 pixels per inch and that always remains the same.
But keep in mind that these "monitor pixels" are only the tiny dots that actually create the "image pixels" you see on the screen.
When the screen is displaying a digital image, these "screen pixels" are used to create "image pixels" that can be almost any size depending on how many screen pixels are devoted to them. For example, when one screen pixel is used to reproduce one image pixel, the image display in Photoshop reads 100% or "Actual size." When two screen pixels are used to display one image pixel the image shrinks to 50%.
Another example of this relationship between fixed and variable image pixels would be the physical holes in the print heads of ink jet printers that spread minute clouds of pigment on paper to reproduce the image pixels we see in prints.
(See Appendix B for more details about pixel size relationships.)
When a digital photograph is either displayed or printed the number of pixels per inch that you see is defined as the image's "resolution." This is one key factor that determines a digital image's quality. For example, if there are 300 pixels in one inch of an image we would consider that to be a high quality image. If there are only 10 we would say the image quality was very poor. Much more on this shortly.
The number of different tones or colors that a pixel is allowed to be in a digital image is a measure of what's known as the pixel's "Bit Depth." This is another key determinant of a digital image's quality.
As you will see from the following examples, learning how to optimize both the resolution and the bit depth of an image's pixels are important skills for digital photographers who want the most from their work.
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