Wot’s A Pixel?

Photographic digital images are built up of pixels.

A pixel is a single block of colour, usually square, and an image is defined as being a number of pixels wide and a number of pixels high e.g 2400 pixels wide x 1800 pixels high or 2400px x 1800px.

A pixel can only be a single colour and no variation of shade or detail can exist within a pixel.

How big is a pixel?

A pixel itself has no linear dimension. Only when WE decide the number of pixels in, say, an inch or a centimeter can the pixels then be regarded as having specific dimensions.
So, taking our 2400px x 1800px image referred to above, if we decide there will be 100 pixels per inch we would be saying that each pixel is one hundredth of an inch square and our image would equate to 24 inches wide x 18 inches high.

How do we decide how many pixels there should be in an inch?

This decision is usually dictated by the use you intend to make of the picture. Will it be a print, a picture on a web page, a large banner etc.

Printing usually requires that image resolution be 300 pixels per inch - indeed, that is the standard adopted by most printing companies for reproduction in magazines, leaflets etc. Desktop printers may produce very acceptable prints with a lower resolution but usually not less than 180 pixels per inch. Images being printed as large banners to be viewed from a distance may be lower still. In general, better print quality requires more pixels per inch are needed although it is not usual to go above 300.

Images intended for viewing on a computer screen or by projection do not require a linear dimension as the screen image itself is defined as a pixel based
area. A common screen resolution is 1024 x 768 which means that an image with these pixel dimensions will effectively fill the screen. An image with smaller pixel dimensions would therefore be somewhat smaller on the screen.