Reader Tony Renshaw emailed wanting to know more about the destructive potential of JPEG compression. He has scanned a large quantity of slides that have been saved as JPEGs and wants to add keyword tags and other metadata to the images.

However, he is concerned that doing this will result in resaving and recompressing of the image data.

Tony doesn’t have anything to worry about; applications that allow you to add and edit metadata in JPEG files only write to the Exif data fields in the file header and don’t affect the actual image data itself.

Not all data may even be written to the file itself –­ ratings and, depending on the application, other metadata may be stored in a separate database.

Adding and editing metadata aren’t the only operations you can carry out on a JPEG file without recompressing the image data. Some applications will allow you to losslessly rotate and flip image files.

In certain circumstances you can even crop JPEG files losslessly, (the top left point of the crop must lie on an 8x8 pixel block boundary –­ see ‘How JPEG works’, below), but not many image-editing applications support this. One that does is Irfanview.

The second part of Tony’s question concerns the amount of degradation involved in JPEG compression, particularly when files are saved multiple times. Although he doesn’t need to worry about this if he’s only editing metadata, it’s an interesting question because it’s good to know what the consequences are for the quality of your images.

Working formats
The first thing to say is that if you’re intending to do a lot of editing on a JPEG file it makes sense to first save it in an uncompressed or lossless compressed format such as Photoshop PSD, TIFF, or PNG.

Then you can save it as many times as you like without having to worry about quality loss, before saving the final output file as a JPEG.