Thursday, October 27, 2011

Ask Tim Grey - JPEG Quality Paradox

Ask Tim Grey - JPEG Quality Paradox

Today's Question: I saw an article today that indicates the quality of a JPEG image saved in Photoshop at Quality setting 7 is actually worse than if you used Quality setting 6. Do you know if this is true? And is it a significant issue?

Tim's Answer: Yes, this is true, but I really don't consider it a significant issue all things considered. As a matter of course I suggest saving with a Quality setting of 8 under most circumstances. If file size is a significant concern to the extent that you want to reduce the Quality setting below that, I do suggest opting for a setting of 6 rather than 7. The two files (saved at Quality settings of 6 and 7) will be about the same file size, but the quality of the image saved at a Quality setting of 6 will be slightly better.

When it comes to quality in a JPEG image, what we generally mean by lower quality is an image with less detail (think of relatively smooth areas taking on an extremely smooth appearance, with virtually no texture) and artifacts (random pixels that don't seem to fit in to their surroundings very well). The differences from one setting to the next are actually quite minor, all things considered. By virtue of the nature of JPEG images (which are always compressed, and thus always involve a compromise in terms of image quality), you're losing some quality from the start. You'll actually have a difficult time seeing a clear difference between images saved at a JPEG Quality setting that is only one value apart, unless you zoom in to the pixel level, at which point you could argue that you're over-scrutinizing a JPEG image. The whole point of this file format is to reduce file size by reducing image quality.

It is worth noting, by the way, that going from a Quality setting of 10 up to 11 or 12 will have a significant impact on file size (especially for 12) without a visible improvement in image quality.

And for those who are interested, the issue of a Quality setting of 6 resulting in a higher quality image than with a Quality setting of 7 has to do with the way compression is applied. In short, up to and including a Quality setting of 6, color values are adjusted to achieve compression. Above that Quality setting, only luminance is adjusted. That would normally result in a larger file size, so to reduce file size adequately the compression must be more extreme, thus resulting in a situation where a Quality setting of 6 results in a slightly higher image quality than with a Quality setting of 7.

But again, this really isn't a big issue, so I don't recommend making dramatic changes to your workflow as a result. Think of it instead as an interesting bit of digital imaging trivia.

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