When taking photographs to be saved as a JPEG or TIFF file, before transfer to a memory card, the digital information is 'processed' by the camera's computer, which takes just a few seconds. After downloading to a computer the processed file can be adjusted in a software programme such as Adobe Photoshop Elements. However, there is a strongly held view that any alteration to a digitally processed image, even in a renowned software programme like the full version of Photoshop, runs the risk of a reduction in image quality, particularly if it is a compressed JPEG.

RAW, which is not an acronym and means what it says, is the preferred picture file for professional use. When saved as a RAW file the photograph bypasses the camera's computer processing, saving it to a memory card unprocessed. After downloading and when opened, the processing takes place in the memory of the computer to produce a full colour image on the screen.

The advantage is that adjustments to exposure, contrast, white balance and many other things can now be carried out in the memory of the computer without damaging the file before being saved as a processed JPEG or TIFF file. The original RAW file remains unaltered and becomes an archival copy. It is like having a film camera where you can make corrections to the photograph before it is processed.

The downside is that the saved file is uncompressed and takes up more space on a memory card and each RAW file requres a 'patch' for every make of camera which the software vendor has to provide before it can be opened on a computer.