Thanks KnightsFan ! It's a little over budget but it's definitely worth considering, I knew there could be more cams with such feature Sadly, I can't use the speed booster as my s35 lenses are old style Arriflex cine lenses, with rear lenses going quite close to sensor. But m4/3 is not much smaller than s35, I could work around...
webrunner5 I think I was maybe not clear enough sorry, I happen to have some cine lenses designed for s35, and some other for the 16mm movie format. I like to use them for different projects, using prime s35 lenses for clips or fiction where I can spend some time working on my cinematography, and using lightweight and more versatile 16mm zoom lenses for a more "documentary" style of project where I have to frame and focus more quickly. Those last zooms cover effectively a very small area (as 16mm films were rather narrow). But it happens that having a 2x "clear zoom" on some APS-C/S35 digital sensors lets you record just the area these lenses can cover. I've done it with a Sony FS5 for a short, letting me use a Zeiss 10-100 vario Sonnar. Without the "clear zoom" option, You'd only get a small image circle on your frame. So yes, "clear zoom" let you use 16mm glass on a s35 sized sensor.
A lot of people used 24x36 or s35 lenses on their BM because most people don't know much about vintage 16mm lenses. But some were far better suited to the camera than anything made for still photography. Another problem is that BM cams have a Super16 sized sensors, slightly bigger than standard 16, but big enough for some zoom to vignette, especially wide open and wide angle. The Sony FS5 at least cropped ideally. But this is a problem I could live with, as they do 4k there's way enough place for cropping in post.
The Sony a7 indeed has a "clear zoom", but it has 24x36 sized sensor and the clear zoom only crops to APS-C, so way too big for 16mm
I hope this is all much clearer to you ?