File Sizes

There are three main factors that determine the size of an image file: pixel dimensions, image format and bit depth.

Pixel dimensions

This refers to the number of pixels in an image: the more pixels that are present, the larger the file size will be. Aim to have an image that is big enough for your purpose but no bigger, as the extra pixel information will not be needed and will only slow down your software. For example, if you wish to show an image in a PowerPoint presentation slide, there is no point in using an image that is larger than 1024 x 768 pixels, as this is the usual size of most computer and projector screens. Similarly, if you wish to show two images on one slide, each image should only be a maximum of 500 x 768 pixels. Even when you resize or crop an image within PowerPoint or Word, the original image information is still retained so although the image appears smaller, it still takes up the same amount of memory. See 'Using Images in PowerPoint' for more information on resizing images and reducing file sizes. For information on software you can use to resize images, look at the suggestions in  Editing images.

Image format

The format of an image file has a large effect on the file size. As discussed earlier, TIFF files are uncompressed and so file sizes are large, whereas JPEG files are compressed, resulting in smaller file sizes, depending on the amount of compression applied. For example, a screen sized TIFF file that is 2 Mb in size reduces to a mere 400 Kb when saved as a high quality JPEG – a fifth of the original file size.

Converting from one format to another can easily be done using any image editing software: again, look at Editing images for suitable software.

Bit depth

Bit depth refers to how many 'bits' of information are used to store colour information in an image file (a bit is the smallest unit of information that a computer can store and it can have two values: 1 or 0, on or off). The higher the bit depth, the more accurately colours will be represented. In the case of black and white images, each pixel contains 8 bits of information to describe the greyness of each pixel. In colour images, each pixel contains 8 bits of information for each of three colour channels (red, blue and green) that make up the colour of each pixel, giving a total of 24 bits (3x8).

As you can see, the more information that is captured about colour, the larger the resulting file size will be. Therefore, if your image is black and white, you should save it in greyscale format as there is no point in recording information about the amount of red, blue and green in the image. This will reduce the file size by approximately a third.

Some scanners capture images at a higher bit depth than the standard 24 bits, but while this may be useful when editing images for colour fidelity, it is not necessary for most uses as the human eye cannot see this extra colour detail. Therefore, ensure that your images are saved in 24 bit colour.

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