If you broke a digital image up into pieces, the smallest single element of that image would be a pixel. In fact, the word pixel is derived from the words picture and element, with a pixel being the smallest possible element in a picture.

When someone speaks of their monitor resolution as being 800 x 600, or 1024 x 728, they are referring to the number of pixels across, by the number of pixels down as displayed on their monitor.

This image has been zoomed in to the point that you can see individual blocks of colour. In a sense, these blocks can be thought of as the pixels (or picture elements) that comprise this image.

In a computer, a pixel is represented as a binary number. (For example, 101011001010110010101100 is the binary number representing a shade of grey). Each 1 or 0 is called a bit. If you have 8 bit colour, than the pixels have 8 bits (8 ones or zeros) to represent a colour. In 16 bit colour, pixels have 16 ones or zeros to represent a colour.

Essentially, the more bits used by a pixel, the more colour can be represented by that pixel. The other side to this is the more bits used by a pixel, the more computer memory is required to store the pixel. This means that higher bit depths (more bits) generally translate into higher quality images with greater sizes.

In a computer, the colour for a pixel is derived from the binary number that represents it. This number is normally interpreted as having three components: a red component, a green component, and a blue component. In a 24 bit pixel representation, each colour would be assigned 8 of the 24 bits (3 x 8 = 24). In the number mentioned above, namely, 101011001010110010101100, the value for each component would be 10101100 for red, 10101100 for green, and 10101100 for blue (10101100, 10101100, 10101100). In this case the numeric values for each component are the same, but this is not usually the case.

These component values tell your computer’s hardware (such as your monitor) how much of each colour to display in the pixel. When the red, green, and blue components are combined, the pixel will display the resulting colour. There will be more discussion of red, green, and blue (RGB) colour in lesson 4.4.

When you edit a digital photo with Photoshop CS, the program manipulates the binary numbers in the image pixels. If you crop an image, you are basically chopping off entire regions of pixels. 

The more pixels that are used in a digital image, the more information the image contains. This implies that in general, more pixels mean higher quality and greater memory requirements. If you have an image with a high bit depth (more bits to represent a pixel) and a lot of pixels (high resolution) your image will also have a large file size.