The Sony e-mount (mirrorless) system has got a few good options for filming long lens. I got the Sony a7s II with the Sony 18-105 f4 power zoom lens (APS-C). The lens has an electronic zoom and zooms very smoothly when turning the zoom ring. Also it has a little zoom rocker on the side of the lens. The lens doesn't extend when zooming. Because it's a powerzoom lens, you can use an external remote plugged in to the camera to control the zoom. I got a GP-VPT1 grip (as shown on the picture) which has a little zoom rocker. I also have a cheap fotga remote (RM-VS1) which also has a zoom rocker. A great combo with the sony 18-105 lens is the Sony a6300 APS-C body, because it has great autofocus for video.