Hi Everybody, Here's another mystery which I finally cleared up.MYSTERY: Why is 4:2:2 the "standard" sub-format for saving JPEGs. Apparently, it is also the standard in Adobe Photoshop but is hidden under the hood.COMMENT: The 4:2:2 sub-format deliberately deletes 33% of the pixels. When a JPEG file is opened, the missing spots are filled in by up-sampling the remaining pixels. QUESTION: Would it not make more sense to keep all pixels by using the 4:4:4 sub-format which keeps all pixels.ANSWER: The 4:2:2 sub-format makes more sense even though 33% of the pixels are discarded. It's all about our visual system. 1. Human vision is far more sensitive to lightness than to color. Thus we should compress more in lightness than in color. 2. This is done in JPEG by first converting the file to the YCbCr color mode. The YCbCr color mode is similar to the Lab color mode in that it has one lightness channel and two color channels.3. For a given JPEG size in bytes, the quality will be better if 50% of the color pixels are discarded rather than applying the same compression to all pixels. It's how we see the world.Phil