The reason I'd like to know this is because of a file size problem. I am trying to reduce an image's file size from converting it from TIFF to JPEG. A user challenge me that he notice a RGB TIFF image is three times the size of the same image but in grayscale TIFF version, but the post-convert grayscale version of the JPEG image is almost the same size as the RGB JPEG version.
He insist that a 8BPP grayscale jpeg should have ~1/3 of the size of the 24BPP RGB jpeg.
Then I have been told by some technical adviser afterward that JPEG will ever only exist in the form of 24BPP file, and that the grayscale jpg images I found online are all lying in their image meta data saying bit depth is 8. But so far I haven't found any proof online that confirm nor deny that claim.
At this point I have no idea if my conversion method is flawed and inflated the file size, or the fact that the so call "8BPP JPEG that takes only 1/3 of the file size" doesn't exist from the beginning.
Is there any documentation regarding this issues online?