A hand- held scanner, used for desktop quality monochrome images or colour scans of small sources, is dragged across the item to be scanned. A slide scanner, capable of high resolution images, records different light levels after the light has passed through the slide being scanned. Before you plan on doing any sort of scanning you need to consider how you want the final image to look and where it will be used. The number of colours, contrast and brightness are a few examples of the features that must be set before the scanning process begins. Dithering is a technique that is used to create the illusion of a third colour using two other colours.
Experimentation is usually the best method of determining the correct settings.
Keep in mind that increasing the size by scaling up will increase the file size of the final image. Changing the image resolution affects the file size and quality of the final image, as well as the scaling range. By increasing image resolution the file size and quality of the final image also increase, but the scaling range goes down. The last thing to consider is that if your final image is only for display on the computer screen, then the display capabilities of that computer are going to determine your scanning settings for the amount of colour and image resolution. If you do not take into account the limitations of your target computer system, the final image will not appear as it does on the computer you are using to scan with, and the file size will be unnecessarily large i.e. there is more information stored about the image than the target computer is capable of displaying.