In an amazing detailed post over at Impulse Adventure, they break down all the different settings of JPEG images and found one interesting little quirk in how Photoshop handles JPEG Images. The moral of the story: Never use JPEG Quality 7.
What many people don’t know is that there is a quirk in the way that Photoshop defines its quality range. As mentioned earlier, 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 (at least from luminance detail perspective).
This fact has apparently been confirmed with subjective MOS scores against various images at both quality levels.
Get all the details at their site.
via ImpulseAdventure – JPEG Quality and Quantization Tables for Digital Cameras, Photoshop. via Gizmodo
