Color/Bit Depth and Image Resolution

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Bit Depth

A bit is the smallest unit of measurement regarding computer data. Each bit is an electronic pulse that can either be on (represented by a 1) or off (represented by a 0). Bit depth refers to the number of colors that can be displayed. The higher the bit depth, the more colors used in the image, therefore, the larger the file size. Note that the maximum number of colors increases exponentially after the original two colors:

Bit Depth	Formula	Number of Colors
1-bit	2^1	2
2-bit	2^2	4
3-bit	2^3	8
4-bit	2^4	16
5-bit	2^5	32
6-bit	2^6	64
7-bit	2^7	128
8-bit	2^8	256
24-bit	2^24	16,777,216

The bit depths most commonly referred to are demonstrated below. Notice how the file size increases as the number of colors increases. Look at what happens when we get to the 24-bit image. Why is that file size smaller than the 8-bit image? It has to do with the file format used with 24-bit images.

Bit Depth	Number of Colors	Sample	File Size
1-bit	2 colors		2 KB
4-bit	16 colors		21 KB
8-bit	256 colors		53 KB
24-bit	16.7 million colors		24 KB

An in-depth view of bit depth

Image Resolution

Resolution is defined as:

... the number of pixels (individual points of color) contained on a display monitor, expressed in terms of the number of pixels on the horizontal axis and the number on the vertical axis. The sharpness of the image on a display depends on the resolution and the size of the monitor. The same pixel resolution will be sharper on a smaller monitor and gradually lose sharpness on larger monitors because the same number of pixels are being spread out over a larger number of inches.

You can think about image resolution as a measure of the output quality of an image. Traditional print work requires high resolution images to display the image quality we are all used to in magazines, brochures, and various other types of print materials.

Images prepared specifically for the Web don't require a high resolution. Computer monitors are limited to an image resolution of 72 pixels per inch (ppi) or 96 pixels per inch; therefore, high resolution images prepared for the Web actually waste a lot of valuable resources (i.e., bandwidth).

Images prepared for the Web at 72 ppi will not look good in print, so it is important to consider the various ways images will be used when determining the desired resolution of an image. There is definitely a trade-off that must be considered: higher resolution images that may be used for print will have a larger file size and require more bandwidth without displaying any better on a computer monitor; lower resolution images may display fine on a computer monitor, but will not be sufficient for print pieces.