On page 14 in the Exif 2.3 specification, under 4.4.3, “Pixel Composition and Sampling”, the following is stated:
“When compressed data is used, the combination shall be Y:Cb:Cr = 4:2:2 or Y:Cb:Cr = 4:2:0.”
Why the heck isn’t a combination of 4:4:4 (i.e. no sub‐sampling) allowed? This will produce the highest quality JPEG compressed images, and many times there is a noticeable difference between using 4:4:4 and using 4:2:2 or 4:2:0. Incomprehensible.