I just purchased a Sirui 50mm 1.8 anamorphic lens, the Sony E-mount version. It fits on the camera fine as expected. However, there is a lot of vignetting. It happens on both 4k and HD recording modes. Is there some kind of camera setting that will fix this? From what I've read this lens should be compatible with the a7s2.
The lens is built for crop-frame [APS-C] cameras, not full-frame. It cannot fill a full 35mm frame, so you get very heavy vignetting.
Unless your camera can shoot video in crop-frame, I'd just send it back.
They do make a FF 50mm anamorphic, but it's a lot more expensive. It's a T2.9 - idk how much difference that is going to make to the final image. https://cf.sirui.com/lens/Anamorphic/fullframe?lang=en-us