Kazuto Yamaki, CEO of Sigma, said in an interview that the Global Vision lenses were checked using their own Foveon sensor.
In the post linked above, I wondered how Sigma could do so by using a sensor that covers only a central portion of the image circle of full-frame lenses. So I amusingly raised the idea of a full-frame Foveon sensor cooked up to that purpose.
In an update to a blog post about their new device dedicated to MTF measurement, Sigma tells more about the process. As a lens is controlled, the Foveon sensor is first placed to assess the central area, and then shifted to cover the other parts of the lens’ image circle. Mystery solved.
On further reading through the post, a more interesting info appears.
100% of the lenses built will be analyzed and approved before leaving the factory in Aizu.
Sigma performs an optical check-up for each manufactured lens, using their MTF measurement tool − a “high-end” quality control, to say the least. The little downside is that it presumably induces a longer time span between the announcement of a lens and its availability, but ultimately it is a positive trade-off.
Reference: A1 MTF Testing for New Sigma Lenses