JPEG quality?

When I export a RAW file as a JPEG, I can enter a "quality" for the JPEG file, a number between 1 and 12. This seems to affect the file very little. What does this number refer to?

Posted on Jul 6, 2014 9:16 PM

Jul 8, 2014 8:03 AM in response to Kirby Krieger

Kirby, that is a great example of the power of the JPEG compression algorithm. JPEG compression is optimised for giving the best visual quality; a compressed JPEG will usually look good and sharp. But the image is no longer the same as the original image, it is an approximation. The details will still be there, but they may shift a bit, the steepness of edges may change, the colours may shift. That is not so important when viewing an image, but it matters, when processing images.

Here is another example: A RAW image (DNG)

We'll look at a section showing the cables of she bridge with different quality settings:

The DNG, quality 5, quality 1, zoomed in at 300 %

You can see, that the edges appear blocky and shift a little, and that the smooth areas will have patches with different colours.

The "quality 1" JPEG, enlarged: The straight lines appear broken and jittering back and forth.

Visually, these image files are still o.k., but image processing is difficult.

Another test: I subtracted the RGB values of "quality 1" JPEG from the RGB values of the DNG images and created a difference images. If the RGB difference is 0, the difference image will be set to a gray value of (128,128,128), otherwise to the RGB-difference times 10 added to (128,128,128).

You can see, that the differences are large along the edges, and that the areas with little structure are showing colour shifts, the hue changing towards purple or green.

This is not quite so noticeable for the difference between the DNG and the "quality 5" JPEG, but there are also differences, mainly along the edges.