Videomaker – Learn video production and editing, camera reviews › Forums › Cameras and Camcorders › DSLR’s › 1st lens for sony A7s › The A7s is an amazing camera
The A7s is an amazing camera with an FE mount which is a Full Frame E mount. You can get an adapter Metabones, Fotodiox etc and use a wide variety of lenses. With the A7R, I've used Leica M, Leica R, the latter being better for the A7R but either will work on A7s. You can also like previous posters suggested get a Canon adapter or Cine lenses. Keep in mind that some reports say that the mount is not that sturdy, so for a heavy lens Fotodiox (I believe) makes a heavier duty replacement mount.
I've been using the A7s (for casual use) for about 6 months. Low light capability is out of this world. In addition to the 3rd party lenses I have have 2 FE lenses for it, the Sony Zeiss 35mm f2.8 and the Sony Zeiss 55mm f1.8. Both are amazing lenses for still and casual video.
You mentioned a preference for a 35mm field of view and therefore I would highly recommend the Sony Zeiss 35mm. Its f2.8 but especially for dance with full frame, anything larger would likely impact your depth of field esp with dancers moving in and out of the plane of focus. The face detection and autofocus work surprisingly well on the A7s though you may intend to use a follow focus or focus manually. The only question I have is whether that Sony Zeiss 35mm has enough ridges to attach a focus gear.
Besides with the Sony A7s's low light capability, you can shoot 12,800 ISO comfortably, so the 1 or 2 stops with a faster lens is moot.
I just pulled the trigger on the Shogun as well, should get it this week and looking forward to playing with it.
Hope this helps.