<p>If the Sonnar T* FE 35mm F2.8 is your maximum size, that's what I would get. The Loxia 35f2 is a great lens but it is more than twice the weight of the Sonnar FE 35f2.8 and only does manual focusing. I have used the Canon EF 35f2 IS on the A7s. It works well, and has the IS, but is bigger than the Sonnar and Loxia, and weighs the same as the Loxia. I just bought the Sonnar and it is very small and light. It also has a unique lens hood that reduces the overall size of the lens in use. So far the pics on the A7s are very sharp with great color, even at f2.8. Autofocus is quick and manual works fine, especially with peaking and magnify active. My initial reason for switching to the Sony mirrorless A7 series was for the weight and size reduction of the camera bodies. The Sonnar FE 35f2.8 is a great lens for a full-frame compact system.</p>