4. So are raw files uncompressed?
Yes and no. Compression is a way of reducing the size of an image using complex maths. The important thing to remember is that there are two types – lossy and lossless.
With lossy compression, as the name suggests, some of the image data is lost during the compression process. However, the files can be made much smaller than is possible with lossless compression.
Some raw formats, such as Nikon’s .NEF files, can be saved in this form, as a way of achieving smaller file sizes, but this can result in lower-quality images, especially if they contain smooth tonal areas, like skies.
All JPEG files use lossy compression. You may have noticed that when you save a JPEG you have to choose a quality setting.
This is because the smaller you save the image the more information is lost, so a high-quality JPEG is a much larger file than a low-quality one.
This quality loss can be acceptable if you only save the image once, but each time you resave the image more data is lost, reducing its overall quality.
Lossless compression retains all of the data in the original file, but the resulting files are often only a little smaller than the original. Most raw formats use this type of compression to ensure that there is no loss in quality when you resave your files. Hence the higher image quality.