Just a half-year after Sony announced the dope A7 and A7r full-frame compact interchangeable-lens cameras it's pushing out a new iteration of the mirrorless shooter. This one's built around video. It does 4K video, too—Sort of. It's not what people were expecting from the A7s, but it might be incredible anyway.
According to Sony, the "s" stands for sensitivity, and it turns out that it's this camera's billboard feature—not on-board 4K video recording like the rumors suggested. The camera can shoot all the way up to ISO 409,600. Impressive by any standards but especially because the A7/A7r only go up to ISO 25,600. The camera also sports a much lower-resolution 12.2 megapixel full-frame sensor. The larger pixels probably account for a large measure of the camera's high-sensitivity.
Why does higher sensitivity matter? It lets shoot in darker conditions at faster shutter speeds. If you don't need very high-resolution—as in the case of video—you can beef up sensitivity by scaling back the number of pixels on the sensor.
As for 4K—the rumors were wrong. The camera does not record 4k. Instead, it's capable of outputting 4K content from the sensor, but you'll have to use external media to record. This gives Panasonic's GH4 a slight edge. Though the GH4 doesn't have the full-frame sensor—it has a much smaller micro for thirds format imager—it'll store 4K on board. For in-camera recording, the A7s uses the XAVC codec Sony developed for 4K.
The other important touch is that the camera can shoot at 120 frames per second at 1280 x 720 resolution, making slow motion video accessible at higher resolution. The one caveat is that the camera only uses an area of the sensor cropped to APS-C-size. That means you'll get a slightly more telescopic video than you would at the full-frame sensor area.
The A7s carries many features from the other A7 models like the amazing electronic OLED viewfinder and the beautiful 2,359,296-dot screen. The pretty much standard features from Sony cameras apply: Wi-fi, easy NFC pairing, etc.
The camera has the exact same dimensions as the A7/A7r, but curiously, it weighs an ounce more. Not a big deal, it's just odd. Pricing and availability have yet to be determined. [Sony]