▁present ▁in ▁the ▁older ▁ 2 8 – 7 0 mm ▁l ens , ▁including ▁poor ▁sharp ness ▁and / or ▁chrom atic ▁aber r ation . ▁According ▁to ▁the ▁head ▁of ▁a ▁US ▁l ens ▁r ental ▁firm , ▁the ▁f / 2 . 8 ▁M k ▁I ▁l ens ▁" def initely ▁has ▁some ▁reli ability ▁issues ▁and ▁a ▁lot ▁of ▁copy - to - copy ▁variation , ▁at ▁least ▁some ▁of ▁which ▁relate ▁to ▁its ▁design ." ▁ ▁Early ▁reports ▁on ▁the ▁f / 2 . 8 ▁M k ▁II ▁indicated ▁significant ▁improvement ▁in ▁optical ▁quality . ▁The ▁a for ement ioned ▁r ental ▁firm ▁head ▁stated , ▁ ▁All ▁versions ▁of ▁the ▁l ens ▁include ▁se aling ▁against ▁dust ▁and ▁water , ▁although ▁they ▁are ▁not ▁water proof . ▁The ▁f / 2 . 8 ▁M k ▁I ▁includes ▁an ▁ 8 - bl aded ▁cur ved ▁dia ph rag m , ▁while ▁both ▁the ▁f / 2 . 8 ▁M k ▁II ▁and ▁f / 4 ▁include ▁a ▁ 9 - bl aded ▁cur ved ▁dia ph rag m . ▁The ▁dia ph rag m ▁of ▁the ▁f / 2 . 8 ▁M k ▁I ▁remains ▁nearly ▁circular ▁from ▁f / 2 . 8 ▁to ▁f / 5 . 6 , ▁according ▁to ▁the ▁Can on ▁Press ▁release ▁from ▁ 2 0 0 2 . ▁Character istic ▁of ▁zoom ▁l enses , ▁it ▁exhib its ▁some ▁bar rel ▁dist ort ion ▁at ▁its ▁short est ▁foc al ▁length