12-30-2017 10:42 AM
Yes, on the EF 24-105mm USM, the EXIF data does confirm that the dot corresponds to 28mm .
Back in the film days, 28mm was a common focal length for a wide-angle prime. (I still have one for my Nikon F-2.) Conceivably that's why Canon thinks it's worth pointing out where that is on the 24-105.
I see that on my 16-35 f/4, the indicated "in between" values are 20, 24, and 28. Maybe Canon uses the dot whenever they want to mark the position of a popular focal length but don't have room for the number.
12-30-2017 08:16 PM
The flag is easy, it marks the actual location of the number representing the FL versus where the number is printed.
The dot, while debatable, is where 28 should be.
How did you determine that? By taking a picture with the lens set on the dot and having the Exif tell you it was at 28? It you're right, maybe the dot is aimed at users who upgraded from the earlier EF 28-105mm f/4.5-5.6. But that's at least as far-fetched as my other guess.
That was my guess . I have no proof but if you take a look at other pictures the OP posted, 28mm is everywhere...Canon deemed it important to mark it. On my 24-70mm f/2.8L II, it was marked 24 (flag), 28, 35, 50, 70.