What are digital images? Digital images are electronic files that contain information about the elements of a picture and how they are arranged. These elements are called pixels (short for picture elements). They arranged in a grid format with each pixel containing information about its colour. Image size refers to the number of pixels in an image, and is measured by the number of pixels along the horizontal and vertical sides of an image, eg 600 x 400 pixels. Image resolution refers to the density at which pixels are displayed: that is, how many pixels are displayed per inch of screen or paper. This is normally measured as pixels per inch, or ppi.

How big will an image appear on my computer screen? Computer screen images are also made up of pixels: the number of pixels depends on the screen’s size and display settings. Most computers with a 17 inch monitor use a display size of 1024 x 768 pixels. You can check your own computer screen by right clicking on an empty part of the desktop, selecting Properties from the drop down menu, and then clicking on the Settings tab.

As a general rule, when a digital image is displayed on a computer screen, one pixel of image data is displayed in each pixel on the screen. If your screen is set to display 1024 x 768 pixels, an image containing 600 x 400 pixels would take up about two thirds of the screen when displayed.

If you had an older computer which had a display area of 800 x 600 pixels, this same image would appear much bigger, taking up over three quarters of the screen, although the image itself still contains the same number of pixels. Images can therefore appear as different sizes on different screens, depending on that screen’s display resolution.

Screen resolution As stated earlier, the resolution of a computer monitor is described in ppi (pixels per inch). Monitor sizes are measured diagonally across the screen from corner to corner.

A 17” monitor with display set to 1024 x 768 pixels 768 pixels 1024 pixels The approximate width of the screen is 13 inches. As 1024 pixels are displayed across 13 inches then the resolution is 1024 ÷ 13 = 79 pixels per inch (ppi) for this typical example.

17”

13”

How can digital images of different sizes be made to fit the screen? An 8 Mega Pixel camera (8MP) will take photos that are composed of about 8 million pixels. This is 3456 pixels wide and 2304 pixels high. A typical 17” monitor will only display 1024 pixels across the screen and so how can the whole picture be displayed on the screen? When a large image such as this is displayed at 100%, just one of the pixels in the original image is displayed on one pixel of the screen. Clearly in this situation the whole image cannot fit on the screen and so you will be viewing only part of the image, as below.

Fitting the whole image on the screen is achieved by a process known as resampling. The software will recalculate the pixels, reducing their number to match the display area and will calculate the colour of the new (fewer) pixels as a sort of average of those from which it was derived. The whole picture is displayed here at a magnification of just 23% of the full size image.

It is possible to make an image permanently smaller by resizing. The resize process involves resampling to a specified number of pixels and then this new picture can be saved. This is sometimes done prior to emailing a picture as it ensures that the picture will always be displayed in full in the email of the recipient. As the number of pixels is reduced the size of the file is also reduced and so the picture can be sent more quickly.