I just did a visual test on three sets of lenses Master-primes, Ultra-primes and S4s. We had, to change the back-focus for the Master-primes a tiny bit, to get the best performance compared to the Ultra-primes and S4s. It was the same on two different cameras. My question is, do the Master-primes in reality require a bit different back-focus or was the Master-prime set we had a bit off (However the rental company confirmed twice that the lenses where checked and are dead on, and all the wider lenses in the sets performed the same way). Can anybody confirm or comment on this phenomenon, please.