Computerized images are made up of pixels, which are small squares of digital information with a colour value. The word pixel is an abbreviation of the expression 'picture element'.

Pixels have no inherent size because their size will depend on the resolution of the image. With two files of the same dimensions, a file with a resolution of 150 pixels per inch will have larger pixels than a file of 300 pixels per inch.

The images in image editing programmes are known as 'bitmap' images, sometimes referred to as raster images.

Drawing programmes, such as Illustrator and Freehand, generate vector graphics which are created mathematically. Vector programmes are best suited for type and strong graphics where sharp edges are required, because when an image is resized, a new mathematical calculation is made and quality is maintained.

When discussing a bitmap image, it should be referred to as pixels per inch (ppi) or pixels per centimetre (ppcm) – not dots per inch (dpi). The image file is made of pixels and the image on the screen is made of pixels. Respectable text books and magazines sometimes talk of dpi here but it is wrong and this term should be reserved for monitors and particularly for printing. In printing, dots of ink are laid down on paper and the two situations need to be distinguished.

So it's ppi for digital images and screen images, and dpi for prints.

The number of pixels per inch needs to be associated with the dimensions of the image in order to give the image resolution. The more data that is recorded by a digital camera or scanner, the higher will be the resolution of the image file.

300ppi in itself is not necessarily a higher resolution than 150ppi; it means nothing until the image dimensions are included.

An image of 150ppi for an image size of 8" x 10" would be written:
8" x 10" @ 150ppi, which is 1200 pixels x 1500 pixels
or 20.3 x 25.4cm @ 60ppcm.

An image of 300 ppi for an image size of 8" x 10" would be written:
8" x 10" @ 300ppi, which is 2400 pixels x 3000 pixels
or 20.3 x 25.4cm @ 120ppcm.

• In this example, 8 x 10 @ 300ppi is going to be a higher resolution than
8 x 10 @ 150ppi.

• However, 8 x 10 @ 150ppi will not be a lower resolution than 4 x 5 @ 300ppi.
The two are exactly the same; they both have a file size of 5.16 megabytes
(5.16 MB).

• The 8 x 10 @ 150 file could be resized to make it 4 x 5 @ 300ppi
and the two will look and reproduce exactly the same.

Summary
The relationship of pixels to area might become clearer
if we compare it to tiling a floor.
With tiles half a metre square, a floor area of 2 metres by 2 metres
would require 16 tiles.
An area of 4 metres by 4 metres would require 64 tiles – four times as many. Pixels and files sizes work the same way.