$ ffmpeg -i in.mp4 -c:v libx264 -crf 18 -vf "eq=gamma=0.5" -c:a copy out.mp4
ffmpeg version N-79341-g4c22441 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.9.2 (Debian 4.9.2-10)
  configuration: --enable-gpl --prefix=/usr
  libavutil      55. 20.100 / 55. 20.100
  libavcodec     57. 34.100 / 57. 34.100
  libavformat    57. 33.100 / 57. 33.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 41.100 /  6. 41.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
Unrecognized option 'crf'.
Error splitting the argument list: Option not found