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Israeli technology company ICVT has developed an method to optimize JPEG compression. The company's JPEGMini system analyses each image to assess the maximum compression that can be applied to an image without loss of perceptible quality. The company says you can expect a 50-80% reduction in filesize over a JPEG that hasn't been intelligently optimized. At present the system can only be used via the company's online service. Meanwhile, the news has prompted blog PetaPixel to reiterate the little-known quirk of Photoshop's JPEG quality slider that means your images may be better saved at quality 6 than 7. (via PetaPixel)
In response to an interview with ICVT on Megapixel.co.il, PetaPixel's Michael Zhang looked into Photoshop's JPEG specs. He discovered that Photoshop's JPEG 'quality' slider isn't as linear as you might think. It's a piece of information that resurfaces from time-to-time, but one we've never covered before so we thought we should explain:
Up to quality level 6, the software conducts chroma subsampling, while at 7 and above, it does not. However, because the full chroma data is so much larger than the sub-sampled version, the chroma and luminance information are then more compressed to stop there being a huge leap in file size as you step from level 6 to 7. As a result it makes more sense to think of the slider as controling file-size, not quality, and to avoid saving at JPEG quality level 7.
Although it's labelled differently, Lightroom appears to use the same 0-12 quality scale as Photoshop, just re-mapped as percentages. As such, it's worth avoiding saving JPEGs at 54-61% in Lightroom.
Photoshop's 'Save for Web' dialogue doesn't behave in quite the same way. Chroma sub-sampling occurs at up to 50% 'quality.' However, unlike normal Photoshop saving, there is a big file-size jump when you move from 50% to 51% because extra compression isn't being performed to maintain a linear file size relationship.