Understanding digital images

This section tries to explain the basics of digital images by answering some common questions. You can either click on an individual question, or scroll down and read through the whole document. Alternatively, you can download the whole section as a pdf file from the link on the right.

• What are digital images?
• If it doesn't have a physical size, how do I know how big an image is?
• How big will an image appear on my computer screen?
• Don't all computer screens have a display resolution of 72 ppi?
• If I want an image to fill a PowerPoint slide, what size does it need to be?
• If I have an image that is smaller than this, can I make it big enough to fill the whole screen?
• Can I make images smaller?
• How can I find out the size of an image?
• How do pixel dimensions and image resolution apply to printing?
• Which file format should I use?
• What factors affect file size?
• Where can I find out more information?

Note: these guidelines concern 'raster' images (sometimes known as 'bitmap' images): images that are composed of pixels. Most photo-realistic images are raster images. Another type of digital image is a 'vector' image, which is composed of lines and co-ordinates rather than dots and is more suited to line art, graphs or fonts. Follow this link for a summary of the differences between these two types of digital images: Vector and Bitmap images.

What are digital images?

Digital images are electronic representations of images that are stored on a computer. The most important thing to understand about digital images is that you can't see them and they don't have any physical size until they are displayed on a screen or printed on paper. Until that point, they are just a collection of numbers on the computer's hard drive that describe the individual elements of a picture and how they are arranged. These elements are called pixels (short for picture elements), and they are arranged in a grid format with each pixel containing information about its colour or intensity.

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If it doesn't have a physical size, how do I know how big an image is?

The physical size of an image when it is displayed on a computer screen or printed out on paper depends on two factors: the image size and the image resolution.

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. This is the easiest (and most accurate) way to think about the size of a digital image: the number of pixels that it contains.

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 often quoted as dots per inch, or dpi, but a more accurate term is pixels per inch, or ppi.

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So how big will an image appear on my computer screen?

Computer screens are also made up of pixels: the number of pixels depends on the screen's size and display settings but most computers these days 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 from the resulting window. (If you are a Mac user, select Display from the System Preferences window, accessed under the Apple menu).

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. So images can appear as different sizes on different screens, depending on that screen's display resolution.    

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But don't all computer screens have a display resolution of 72 ppi?

Although you may have heard that computer screens have a resolution of 72 ppi, the actual resolution varies from computer to computer, depending on the size of the screen and its display properties; resolutions of between 80 and 100 ppi are more common these days. You can work this out by dividing the display size in pixels by the physical size of the screen in inches. For example, my own screen measures 13 x 9.5" and has a display area of 1024 x 768 pixels, giving a display resolution of 78 ppi. However, a colleague's screen measures 17 x 10.5" and displays 1680 x 1050 pixels, giving a resolution of 98 ppi.

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If I want an image to fill a PowerPoint slide, what size does it need to be?

This will depend on the display settings of the screen or projector that the presentation will be displayed on. Most monitors and projectors these days are set to 1024 x 768 pixels, so you should aim to have an image size of that size in order to fill a whole screen. Further details about using images in PowerPoint can be found in Using digital images.

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If I have an image that is smaller than this, can I make it big enough to fill the whole screen?

Remember how an image is displayed, with one pixel of image data per pixel of screen? If you try and make a smaller image larger, the resulting image will lose quality because there is not enough image data for the number of pixels on the screen. The computer then has to try and fill in the blank spaces itself and it does this by copying image data from the adjacent pixels, resulting in a blocked effect, called pixelation, as shown here. To avoid this, always try and obtain an image that is big enough for your intended use.

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Can I make images smaller?

Yes, you can make an image smaller if it is too big for your requirements. When you click on an image in PowerPoint or Word, resizing handles (small dots) appear at the corners of the image and you can drag these inwards to reduce the size of the image: don't use the handles at the sides or top of the image as these will squash and distort the image, as shown in this example. Further information on resizing images is given in Using digital images.

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How can I find out the size of an image?

There are a number of ways to find out the size of an image (its pixel dimensions):

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How do pixel dimensions and image resolution apply to printing?

For printing, image resolution refers to the number of pixels that are needed to print each inch of the image. For example, ink jet or laser printers generally need to have around 150 ppi in order to produce a reasonable image on plain paper, and up to 300 ppi for photo quality printing.

Therefore, you need to decide how large you want the image to appear on the printed page (in inches), and multiply this by the required resolution (150 ppi) to determine how many pixels you need to get a good quality print. For example, a printed image of 6" x 4" (about half an A4 page) would require 900 x 600 pixels (6 x 150, and 4 x 150). See Using digital images for further details on how images can be incorporated into text documents.

If you are preparing images for publication (eg publicity material or to illustrate an article), contact your publisher to find out what size of image they require as they may need a higher quality image.

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Which file format should I use?

There are four main file formats that you may come across when finding or creating images: TIFF, JPEG, GIF and PNG. Generally speaking, if you are working with photographic or other continuous tone images, you should be using JPEGs when incorporating images into teaching and learning materials because they are of good quality with a small file size. They are also the most common image file format found on the web.

TIFFs, while they have excellent quality, generally have a very large file size which may make presentations run slowly, and GIFs have limited colour capabilities and so are more suitable for icons or very simple images. PNG is a newer format that share characteristics with both TIFFs and JPEGs, but it is still not a very common format.

For more information on different formats, have a look at Image Formats.

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What factors affect file size?

File size is affected by three factors: pixel dimensions, image format and bit depth. In general, reducing your pixel dimensions to only what you actually require, using the JPEG format and setting the lowest possible bit depth will give you the smallest file size. You can find out more about how each of these factors affects file size by looking at File Sizes.

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Where can I find out more information?

If you would like to learn more about digital images, there are many web sites that give further information. The following sites are a good place to start:

JISC Digital Media

JISC Digital Media provides advice and support in the use of digital images in higher education. Their web site has lots of information and advice on creating and using digital images in teaching and learning. In particular, the following pages may be of interest in understanding digital images:

Scantips,com

This site has lots of information relating to digital images and scanning, and although it can be a bit wordy and repetitive, it is very good at explaining the fundamentals. In particular, the following pages give a good introduction to the basics:

All About Digital Photos

Although this is part of a personal web site, it does offer some easy to understand explanations about digital images. In particular,  The myth of DPI may be helpful in understanding image size.

Cornell University Library: Digital Image Tutorial

This tutorial on digital images offers some 'base-level' information on digital imaging as well as a few quiz questions to check your understanding. The section on Basic terminology is particularly useful in explaining the basics of digital images.

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