AF stand for Auto-focus and it is printed on lens barrel. It describes lens capability to work with camera body to achieve focus automatically. Details about AF systems and how they work can be found here.
Source:APO is acronym for Apochromatic used by Sigma to denote that lens has been made using special low-dispersion (SLD) glass and is designed to minimize color aberration. Usually contained in lens name eg. (150mm F2.8 EX DG OS HSM APO Macro)
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