ffmpeg -vcodec dnxhd -i 1080185.mxf -vcodec dnxhd -b:v 185 -an test.mov
ffmpeg version N-38624-gc088b7f Copyright (c) 2000-2012 the FFmpeg developers
  built on Mar  8 2012 08:34:23 with clang 3.1 (tags/Apple/clang-318.0.54)
  configuration: --enable-nonfree --enable-gpl --enable-version3 --enable-postproc --enable-swscale --enable-avfilter --enable-hardcoded-tables --disable-shared --enable-pthreads --disable-indevs --cc=clang --disable-mmx --enable-ffplay --enable-static
  libavutil      51. 42.100 / 51. 42.100
  libavcodec     54. 10.100 / 54. 10.100
  libavformat    54.  2.100 / 54.  2.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 63.100 /  2. 63.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  7.100 /  0.  7.100
  libpostproc    52.  0.100 / 52.  0.100
[dnxhd @ 0x7fb3f883fc00] frame size changed: 1920x540 -> 1920x1080
Input #0, mxf, from '1080185.mxf':
  Duration: 00:00:07.56, start: 0.000000, bitrate: 193348 kb/s
    Stream #0:0: Video: dnxhd, yuv422p, 1920x1080, 25 fps, 25 tbr, 25 tbn, 25 tbc
    Stream #0:1: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
    Stream #0:2: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
    Stream #0:3: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
    Stream #0:4: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
    Stream #0:5: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
    Stream #0:6: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
    Stream #0:7: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
    Stream #0:8: Audio: pcm_s24le, 48000 Hz, 1 channels, s32, 1152 kb/s
File 'test.mov' already exists. Overwrite ? [y/N] y
w:1920 h:1080 pixfmt:yuv422p tb:1/1000000 sar:0/1 sws_param:
[dnxhd @ 0x7fb3f8848000] video parameters incompatible with DNxHD
Output #0, mov, to 'test.mov':
    Stream #0:0: Video: dnxhd, yuv422p, 1920x1080, q=2-1024, 0 kb/s, 90k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (dnxhd -> dnxhd)
Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height