JPEG optimization. Part 3 July 5, 2006

Every time you specify image compression quality name the program you are using for optimization. The thing is that JPEG standard does not have strict rules—it only determines how the image is allowed to be transformed in order to reduce file size. But it is the developer who decides what exactly the optimizer will do.

For example, some marketers position their products as the best optimization software that enables to save smaller files with high Quality settings whereas with Photoshop you’d get files two times as heavy. Do not get taken in. Each program has its own Quality scale, and there are different values that impose additional optimization algorithms (I’ll touch upon this later).

The only criterion to compare optimization performances is quality to size ratio. If you save an image at quality of 55-60 using Photoshop, it will look like and have the same size as files made with other software at 80 quality.

Never save images at 100 quality. This is not the highest possible quality, it is only a mathematical optimization limit, and you will end up with an unreasonably heavy file. A quality of 95 is more than enough to save the image with practically no losses.

Keep in mind that when you set quality under 50 in Photoshop, it runs an additional optimization algorithm called color downsampling. What it does is it averages color in the neighboring 8-pixel blocks:

48×48, quality 50, 530 bytes

48×48, quality 51, 484 bytes

So, if there small high-contrast details in the image, it is better to set a quality of at least 51.