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Chroma subsampling was developed in the 1950s by Alda Bedford for the development of color television by RCA, which developed into the NTSC standard; luma-chroma separation was developed earlier, in 1938 by Georges Valensi.
Through studies, he showed that the human eye has high resolution only for black and white, somewhat less for "mid-range" colors like yellows and greens, and much less for colors on the end of the spectrum, reds and blues. Using this knowledge allowed RCA to develop a system in which they discarded most of the blue signal after it comes from the camera, keeping most of the green and only some of the red; this is chroma subsampling in the YIQ color space, and is roughly analogous to 4:2:1 subsampling, in that it has decreasing resolution for luma, yellow/green, and red/blue.
@CJD @skutumpah @Richard_P
So fascinating and so technical I'm studing for a couple of hours every day online with Damiensymonds.net to increase my Photoshop skills. Only this morning I was reading about the reasons for shooting in RAW. Evidently the main reason is that every speck of light that hits the sensor is caputured - whereas Jpeg can only contain a limited range of tones (because of compression I suppose) - Quite aggressive adjustments can then be made to a raw file with no loss of quality. Damien prefers Adobe Camera Raw.
He also believes that a high quality Jpeg is just beautiful despite the compression issue.
I agree that having a raw file of your artwork is very important and I hope that in a few months' time I will know how to adjust the images in ACR and then in Photoshop to my heart's content. I have a huge amount of photo restoration to do. I must have made some progress since my initial post about Jpegs in October last year.