"There are three versions of this lens. The original lens has a five-blade diaphragm. Partway through its production run, this was replaced with a seven-blade diaphragm; however, contrary to normal Canon practice, the name of the lens was not changed. The third version, the EF 28-105mm f/3.5-4.5 USM II, is a cosmetic change. Apart from the diaphragm, all three versions have identical optics and mechanics."

Zrodlo: http://en.wikipedia.org/wiki/Canon_EF_28-105mm_lens

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