That's also a question of mine ...
Kubrick used to test immidiatelly every new lens that came out ... So may he preffered the b speedlook that the mkiii??
1. ... (from ACM) Smith reports that Kubrick actually asked DiGiulio to recalibrate the Barry Lyndon lenses for use on Eyes Wide Shut, but eventually scrapped that plan. "Stanley wanted to shoot with available light and real sources wherever possible," Smith relates. "We discussed the idea of using the f0.7 lenses from Barry Lyndon, but they just weren’t right for the type of shooting we were doing. Stanley wanted to be able to show some of the sets, such as the ballroom in the opening party sequence, in 360 degrees, via extensive Steadicam work and wide-angle lenses. He wanted to give the actors the flexibility to move wherever they needed to, and he also wanted to swing the camera around the room without worrying about where the lights were. Furthermore, Barry Lyndon was made more than 25 years ago, when film stocks were rated at 100 ASA. Now we have the luxury of 500- and 800-speed stocks, which eliminates the need for specialized lenses like those old f0.7 Zeisses."
So may the mkiii upgrade was a compromise just to have the extra 0.1T ? .On the other hand I have study many of EWS daily continuity reports .. and even on external night scenes the maximum aperture came across is T1.4 ... never a T1.3 !! (May did not want the lens fully opened ?!?!?)
2. Maybe the upgrade was becouse he found out that the mkiii match up better with the Variable Primes ?? (Also from the reports I have noticed that on shots with smaller than 2.1T and focal length available on the super speed scale .. he usually PREFFERED the Variable Primes (e.g. 18mm - 35mm - 65mm)