For print
The process of printing reproduces images using microscopic dots of ink. In 4-colour or ‘CMYK’ (cyan, magenta, yellow and black) printing, dots are printed in each of the 4 colours at different densities and in rows at different angles so that the colours effectively combine in our vision to form continuous colour. The finer the resolution – that is, the greater the number of dots per inch (dpi) – the better and more accurate or lifelike the printed image will look.
Printed inks can create tens of thousands of shades – close to the human range of visual perception – but the results depend heavily on the paper or other surface on which the image is printed.
Photographs for printed publications need to be supplied as high-resolution electronic files so that they can be placed on the page at 300 dpi. Images for large-format printing (eg banners, large posters) can sometimes get away with 150–200 dpi if the quality of the image is particularly good, but it is still best to supply high-resolution images wherever possible. This will give the greatest flexibility in deciding how to crop and place images on the page. Images for CMYK printing should be supplied and placed as CMYK format.
Caution! The minimum resolution of image files for printing is 300 dpi. Check the resolution and size of the images. Many images that look good on a computer screen will not be adequate for a printed document. If they are too low in resolution, they may have to be placed at an unsatisfactorily small size on the designed page, or may look jagged and pixelated.
File size is not necessarily a clear indicator of image quality, which also depends on the file format and level of compression (as well as the quality of the composition, lighting and focus, etc). As a rough guide, a high-quality JPG file size should be at least 2 MB for even a ¼ A4 page–sized image.
The photographs below show examples of different quality images. The photograph on the left is highly compressed (too low in resolution), resulting in pixelation, while the one on the right is not. Note that the focal range is very short, allowing the subject to be crisply in focus while the foreground and background are out of focus and thus less distracting.
Photograph at low (left) and high (right) resolution
To find out the resolution of your image, open the file with an image editing program such as Microsoft Photo Editor, an image management program such as Adobe Bridge, or image processing software such as Adobe Photoshop. The image type, resolution and size are usually found via the file or image properties menu item.
Do not increase the resolution of the image by resampling it – resampling adds extra pixels by interpreting the adjacent pixels. It does not create or restore image data and will result in a fuzzy image.
For screen
A screen is packed with a certain number of pixels per inch (ppi) – for most computer screens, this is 72 ppi – in a fine mesh, each of which shows either red, green or blue (RGB) light at varying intensity.
Onscreen colours are limited by the combinations of red, green and blue pixels. It is almost impossible to accurately match an onscreen colour with a printed colour because of the difference in vibrancy and colour mixing methods.
Images for screen- and web-only use can be supplied or placed in a document with RGB values in a range of file formats. The minimum resolution of image files for onscreen viewing is 72 dpi.
