[...] sometimes you increase the quality setting in Photoshop and the actual quality of the image is reduced…
I had never heard of that before, so I decided to dig a little deeper.
According to an article cited in the interview, there’s a fundamental change that happens between the values 6 and 7 when choosing the JPEG save quality in Photoshop:
What many people don’t know is that there is a quirk in the way that Photoshop defines its quality range. [...] Quality level 6 is the last point in which chroma subsampling is used. At Quality level 7 and higher, no chroma subsampling is used at all. With the amount of color information encoded now doubled, the file size would have naturally increased significantly at this level versus the previous level.
However, it is likely that Adobe decided to allocate the various quality levels with some relationship to the final compressed file size. Therefore, Adobe chose a poorer luminance and chrominance compression quality (i.e. higher level of compression) in Quality level 7 than Quality level 6! What this means is that the image quality of Quality level 7 is actually lower than Quality level 6.
Did you catch that? What they’re saying is that there’s a different compression strategy is used for a quality of 7 compared to 6. Holding the JPEG image quality constant, this change would normally drastically increase the file size of the photograph. However, not wanting to have a giant leap in file size between 6 and 7, Adobe actually reduces the image quality when going from 6 to 7 in order to compensate for the larger file size!
So basically, don’t ever use a quality of 7 when saving JPEGs with Photoshop. Either use 6, or something higher than 7 if you want to actually increase the quality of the photo.
JPEG Compression Quality from Quantization Tables [ImpulseAdventure]
P.S. Another interesting fact found in the article is an explanation of why the quality values go from 1-12 instead of the standard 1-10. They say it’s because the maximum value people are expected to use is 10. For the values 11 and 12 (included for “experimental reasons”), you don’t actually get much noticeable change in image quality, but file size balloons like crazy!