Sony's a7 and a7r full-frame mirrorless cameras were only so-so for video, but the a7s is an absolute killer. The only thing lacking at launch was a good selection of lenses to suit the needs of videographers. That is starting to change with the new 28-135mm f/4 OSS.
Designed specifically for videographers, the 28-135 f/4 OSS is a power-zoom lens for smooth near-to-far transitions. It's a burly sucker, but should be a really nice all-around documentary lens which a good zoom range and stabilization to keep the shakes at bay.
It has three separate control rings for aperture, zoom, and focus, along with special guts that enable quite zoom transitions, as well as smooth click-less aperture adjustments. It's a weather-resistant as well. The last subtle-but-really-fancy feature is this lens' ability to focus without shifts in focal length. Normally when you focus a lens, the moving elements cause subtle shifts in its field-of-view. It's not a big deal for still shooters, but for video shots that rack focus from one object to the next, it has an undesirable look. The architecture of Sony's lens reduces this effect, known as focus breathing.
This all sounds well and dandy until you notice that this thing weighs 2 lbs 11 oz. Dear. Lord. Might not be so comfortable going hand-held with this one. No wonder it has a tripod mount. Then there's the $2500 price-tag which might make you gasp. But actually this type of cinema-geared lens is usually an insane amount more than that, so consider this a pretty darn cheap option.