Over the years, I've standardized on a JPEG compression factor of 15; I find that generally provides the best compromise between image quality and file size for most photographic images.
Although I've done some ad-hoc testing that pointed to compression factor 15 as the sweet spot before, I've never done a formal test. So I performed a JPEG compression series using the Lena reference image*. Note that I resized the image slightly (from 512x512 to 384x384) to keep the file sizes relatively small. The original, uncompressed image size is 433 kilobytes.
|compression factor 10 (39 kb)||compression factor 15 (30 kb)|
|compression factor 20 (26 KB)||compression factor 30 (16 KB)|
|compression factor 40 (11 KB)||compression factor 50 (9 KB)|
Beyond 50 percent compression factor, quality falls off a cliff, so I won't bother displaying anything higher. Here's a more complete breakdown of JPEG compression factor and file size for the 384x384 Lena image:
I was also curious what the image quality and file size penalty was for recompressing a JPEG image. That is, opening a JPEG and re-saving it as a JPEG, including all the artifacts from the original compressed image in the recompression. I've been forced to do this when I couldn't find an uncompressed or high quality version of the image I needed, and I always wondered how much worse it made the image when I recompressed it.
For the recompression test, I started with the uncompressed, resized 384x384 Lena reference image. For each new generation, I opened and saved the previous generation with my standard JPEG compression factor of 15.
|Generation 1 (30kb)||Generation 2 (30kb)|
|Generation 3 (30kb)||Generation 4 (30kb)|
|Generation 5 (30kb)||Generation 10 (30kb)|
I was quite surprised to find that there's very little visual penalty for recompressing a JPEG once, twice, or even three times. By generation five, you can see a few artifacts emerge in the image, and by generation ten, you're definitely in trouble. There's virtually no effect at all on file size, which stays constant at 30-31 kilobytes even through generation 15.
* An entire set of classic reference images is available from the USC-SIPI image database. I distinctly remember that Mandrill image from my Amiga days.