Recently released 50-500mm OS (which I´m currently reviewing) and the 150-500mm OS plus the 120-400mm OS are witin a limited series having some focus issues in certain situations. Contact your local Sigma dealer in order to get it checked and fixed.

Sigma writes: “After thorough investigation, we have ascertained that some APO 50-500mm F4.5-6.3 DG OS HSM, APO 120-400mm F4.5-5.6 DG OS HSM and APO 150-500mm F5-6.3 DG OS HSM lenses, produced recently, may have a potential autofocus defect.

This issue could possibly occur in some lenses within the following range of serial numbers.

[List of lenses requiring the modification]

APO 50-500mm F4.5-6.3 DG OS HSM
APO 120-400mm F4.5-5.6 DG OS HSM
APO 150-500mm F5-6.3 DG OS HSM

Serial Number between 10633051 and 10972000.
Serial Number between 10574001 and 10972000.
Serial Number between 10674301 and 10972000.

This is a limited problem that will not occur in any lenses other than those specifically listed above.

[Support for this problem]
To overcome this issue, we will be offering our customers an inspection and where applicable, a modification service, free of charge. In the case of any autofocus problem becoming apparent on inspection, we will rectify the situation through the help of our local authorized Sigma Service Stations.”

Read more about the issue HERE