Q Like most of your readers with digital cameras, my photographs are saved in JPEG format. I find .jpg is also the most practical and economical format for storing lots of scanned pictures.

We are often told that every time a JPEG file is opened, some loss of quality occurs. What I should like to know is what constitutes opening the file? Does this loss of quality occur when we look at thumbnails in My Pictures, or when we go through them in Filmstrip view?

Clearly this is a huge worry if I want to archive all my pictures and be able to view and show them regularly. Are JPEG files stored on CD-Rs still subject to this deterioration when viewed?
Simon Slater

A Deterioration to a JPEG image occurs not when it is opened, but when it is saved. You’re perfectly safe opening any picture, as long as you don’t save it again. Obviously, if the program you use can only view images (and not save them) you’re safe.

That applies to viewing the thumbnails of any image in Windows or another program, or viewing the image using the Filmstrip in Windows Explorer. And if your photos are stored on a CD, most of the time it will be impossible to resave the image to the CD, so you’re quite safe opening any image from a CD in any program.

Things are a little more complicated if you open the file with a photo-editing program. As long as the program does not automatically save your files, and you don’t manually save them, no changes will be made to the pictures, and there will be no deterioration.

Even if you do save the file, there is likely to be little deterioration as long as you haven’t made any changes. The drop in quality occurs because the file is compressed each time you save it. At this point the computer throws away all the information in the image that it thinks isn’t visible to the human eye, and if it miscalculates this it could cause a drop in quality.

If you are going to edit an important file, it is a good idea to save it in a format that will not deteriorate as it is saved. This will allow you to save regularly. A good file format is TIFF. These files are bigger than JPEG files but the image can be saved as a JPEG when you have finished editing.