Pixels and resolution

When digital technology is used to capture, store, modify and view photographic images, the images must first be converted to a set of numbers in a process called digitisation. Computers are very good at storing and manipulating numbers and can therefore handle digitised images with remarkable speed. Once digitised, photographs can be examined, altered, displayed, transmitted, printed or archived in an incredible variety of ways. As you explore digital imaging, it helps to be familiar with a few basic terms.

Digital images consist of a grid of small squares, known as picture elements, or pixels: These basic building blocks are the smallest elements used by computer monitors or printers to represent text, graphics, or images.

Resolution describes the clarity or level of detail of a digital image. Technically the term "resolution" refers to spatial resolution and brightness resolution; commonly, however, the word is used to refer to spatial resolution alone. The higher the resolution, the greater the detail in the image (and the larger the file). For computers and digital cameras, resolution is measured in pixels; for scanners, resolution is measured in pixels per inch (ppi) or dots per inch (dpi); for printers, resolution is measured in dots per inch (dpi).

Bit depth

Bit depth, sometimes called "brightness resolution", defines the number of possible tones or colours every pixel can have. The greater the bit depth, the greater the depth of colour, and the larger the colour (or greyscale) palette (number of colours). For example, 8-bit colour has a range of 256 colours (or shades of grey) and 24-bit (or higher) colour provides 16.7 million colours, but 30-bit colour has many more millions of colours, which offers higher definition and thus better results in reproducing details such as the shadowy parts of an image.

24-bit true colour

8-bit 256 colour

8-bit 256 greyscale

2-bit black & white

Files and file formats

Once you take a digital photograph or scan an image, the image becomes a digital file, which is stored on a computer like any other file. Image files tend to be larger than text files, since images require more data to represent visual information. The file size of an image (usually measured in megabytes, or MB) indicates the amount of data required to capture a specific image to a given level of resolution. Digital image files are stored in a variety of file formats, some specific to a given type of computer or software.

A digital image is stored as a computer graphics file allowing it to be viewed, printed, transmitted and edited. There are two basic forms of computer graphics: bit-maps and vector graphics. Nearly all digital images are stored as bit-maps and there are four bit-map file formats that you can use: JPEG, GIF, TIFF and BMP. When deciding which of these you will use to store your digital images, it is important to consider the pros and cons of each:

See more information on compression, colour modes, vector and bitmap formats.

Exercise

Now that you better understand concepts like resolution and file format, it is time to consider how they might relate to the different types of collection materials you wish to digitise. What follows is a guide to the kinds of conversion methods and equipment, along with resolutions, you should consider as part of your preparation and research. This chart will help you decide where you should concentrate your resources. If the bulk of your collection consists of colour maps or illustrations then it may well be more cost effective, and less time consuming, to use a flatbed scanner capable of producing 24-bit images at 300 dpi. Use the chart below to analyse what your institutional needs are. Once you have established your priorities you can then look at what equipment is available, its cost or indeed alternatives like contracting out.

Table for analysing possible media types and resolution recommendations:

Do you understand:

• The meaning of the terms pixel, bit depth and resolution?

• What file formats are available?
• The pros and cons of all the different file formats?

Date published: 1 August 2001