Image resolution explained

Resolution is the amount of information included in the graphic file and this determines the quality of the image. Resolution is measured in Dots per linear inch (or dpi) (although strictly speaking, pixels per inch is ppi but dpi is used in its place).

The higher the resolution, the larger the file size and the better the quality image. The image looks better with higher resolution because more information is contained in the file that the computer uses to reproduce the image either on paper or on the screen. However, your monitor cannot display above 72 dpi, so anything above 72 dpi will be wasted.

The challenge is to create images that contain only the essential information the computer requires but at the same time acceptable quality to the human eye (optimisation). Mostly you will find that the human eye is very forgiving and your audience will perceive the purpose of the image equally well whether it has been saved at high or medium resolution. What will not impress your audience is if they have to wait too long for the image to appear!

Print resolution

Printed images are created using an arrangement of dots on paper.
The smaller and finer the dots, the better quality image that will be printed.
Your standard desktop printer would be capable of printing at least 300 dpi.
A high resolution output devise, for glossy magazine quality photographs, may print up to 2540 dpi or more.

Screen (monitor) resolution

Your monitor is capable of presenting only 72 dots per inch. Although this may seem to be very low resolution, it is actually quite adequate for human eyes.

Bit resolution (Bit depth or Colour depth)

Bit resolution simply describes how many colours a pixel is capable of displaying.

  • 1 bit pixels are capable of displaying one colour value (ON or OFF) black & white.
  • 8 bit pixels can display one of 256 possible colour values (28)
  • 16 bit displays more than 65,000 colours
  • 24 bit displays more than 16 million colours

A graphic can be saved to display a certain pixel depth. Video cards and monitors have fixed pixel depths and cannot display more colours than their bit depth allows.
For example, if you set your monitor to 8-bit, it will only be able to display images at a fixed capability of 256 colours. Your graphic may have been saved at 24 bit depth (millions of colours) so the monitor must try to present all these colours using only the 256 colours it is capable of displaying. This results in a loss of colour quality in the graphic.

Dithering

When an image must be presented on screen at a lower bit depth than it was saved, the image may be “dithered”. This is where the monitor tries to give the effect of the original colour that it cannot match exactly, by using a combination of 2 or more colours that it is capable of showing. Dithering relies on the human eye to blend the separate colours into one shade of colour.

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