Just for fun I thought I'd see how the larger catalogs fair against a picture. The animated GIF shows
1. the VLT image; followed by
2. PPMXL+UCAC3 limited to Mag 15 on Cartes Du Ceil; followed by
2. PPMXL+UCAC3 limited to Mag 20 on Cartes Du Ceil
For some reason PPMXL on it's own wasn't showing all the stars in the interior of the cluster, which is why I used both. Alignment is a little sloppy as you can see, but it's late and I'm tired. I think I've seen as faint as magnitude 14.6 in my 12" but it's only a couple of weeks old so I'm basing that on just 2 or 3 sessions and a very rough guess based the magnitude listed for the faintest star I could make out.
http://www.progsoc.uts.edu.au/~sammy/astronomy/PleiadesESOVLTCartesDuCielOverlayMa g15Mag20.gif