Some people have the same problem, but here is the solution.First, if you have more than 1 GPU, it's normal: Thief does not support (as of March 2014) multi-GPU. Maybe there was an update since then.Of course, you have to check if you're using the latest drivers, and also if you uninstalled the previous one, to be sure they're not conflicting.A user on a Steam community thread also said this:Mantle seems to be incompatible with the setting 'Depth of Field' set on 'normal'.

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• Sims 4 64 Bit Patch

Sims 4 64 Bit Patch

Just use the 'High' setting.Here is the source.