On page 14 in the Exif version 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 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 (especially in red areas) between using 4:4:4 (no sub-sampling) versus using 4:2:2 or 4:2:0. And Photoshop happily saves Exif files with no sub-sampling when using higher quality levels. I have already mailed CIPA in Japan about this, but I'm not very hopeful about a response from them.
We can't do much about the EXIF specification, we just work with it as best we can.
You have to remember that EXIF is written by camera makers, who are more interested in compressed JPEG images than high quality (more photos on a card).
Wikipedia has an excellent article on JPG. Best idea is for you to read it, and come back and say you now understand it and am completely happy. The alternative is that our Noel and other folk will get into a long and pointless argument, when they have other things they should be getting on with.
It is a very good article though.
Forget everything about what I said. I can't expect to get any sensible help from a bunch of amateur Photoshop aficionados, what am I thinking? I'll have to wait for a possible reply from CIPA. Anyway, thanks for showing off.
Good gracious. Have you ever heard of time zones, or that folk have work to do? Do you even realise who Chris is? I am sorry for my glib and factious reply, but I was trying to warn you that pixel-peeping type threads tend to end in tears. I was also being completely serious about the Wikipedia article. It’s an excellent resource, and has a ton of reference links.
There's no use being snotty, Trevor. I can't get any information about why CIPA has decided that using no sub-sampling in JPEG Exif files is disallowed by looking at a generic article about JPEG. Logical, right? That's why I posed the question here. To no avail, it seems. I have sent Chris a private message.