What do you think about this rendering profile for a camcorder source with 1080p and 50 fps (Panasonic HC-V727)
FX-8320 cpu (8 cores)

acodec=ac3 ac=6 ab=448k ar=48000 vcodec=libx264 vpre=libvpx-1080p50_60 crf=20 threads=9

The result then is:

mediainfo test.mkv
General
Unique ID : 127697614502082335346218316057784467735 (0x6011AA7C54B0DB33E8297496D389C117)
Complete name : test.mkv
Format : Matroska
Format version : Version 2
File size : 45.6 MiB
Duration : 56s 620ms
Overall bit rate : 6 755 Kbps
Writing application : Lavf53.21.1
Writing library : Lavf53.21.1

Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : Main@L4.2
Format settings, CABAC : Yes
Format settings, ReFrames : 3 frames
Codec ID : V_MPEG4/ISO/AVC
Duration : 56s 620ms
Bit rate : 6 172 Kbps
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 50.000 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.060
Stream size : 41.7 MiB (91%)
Writing library : x264 core 120 r2151 a3f4407
Encoding settings : cabac=1 / ref=3 / deblock=1:0:0 / analyse=0x1:0x111 / me=hex / subme=7 / psy=1 / psy_rd=1,00:0,00 / mixed_ref=0 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=0 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=-2 / threads=9 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=3 / b_pyramid=0 / b_adapt=1 / b_bias=0 / direct=1 / weightb=0 / open_gop=1 / weightp=2 / keyint=250 / keyint_min=25 / scenecut=40 / intra_refresh=0 / rc_lookahead=40 / rc=crf / mbtree=1 / crf=20,0 / qcomp=0,60 / qpmin=0 / qpmax=69 / qpstep=4 / ip_ratio=1,25 / aq=1:1,00
Default : Yes
Forced : No

Audio
ID : 2
Format : AC-3
Format/Info : Audio Coding 3
Mode extension : CM (complete main)
Format settings, Endianness : Big
Codec ID : A_AC3
Duration : 56s 620ms
Bit rate mode : Constant
Bit rate : 448 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Bit depth : 16 bits
Compression mode : Lossy
Stream size : 3.02 MiB (7%)
Default : Yes
Forced : No

As an alternative with the same options, but crf=22 (a little bit lower quality)

acodec=ac3 ac=6 ab=448k ar=48000 vcodec=libx264 vpre=libvpx-1080p50_60 crf=22 threads=9

I get

mediainfo test2.mkv
General
Unique ID : 175277674106133390991087094503118999139 (0x83DD408A90FD6039C3E09A0E91280E63)
Complete name : test2.mkv
Format : Matroska
Format version : Version 2
File size : 32.3 MiB
Duration : 56s 620ms
Overall bit rate : 4 789 Kbps
Writing application : Lavf53.21.1
Writing library : Lavf53.21.1

Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : Main@L4.2
Format settings, CABAC : Yes
Format settings, ReFrames : 3 frames
Codec ID : V_MPEG4/ISO/AVC
Duration : 56s 620ms
Bit rate : 4 246 Kbps
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 50.000 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.041
Stream size : 28.7 MiB (89%)
Writing library : x264 core 120 r2151 a3f4407
Encoding settings : cabac=1 / ref=3 / deblock=1:0:0 / analyse=0x1:0x111 / me=hex / subme=7 / psy=1 / psy_rd=1,00:0,00 / mixed_ref=0 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=0 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=-2 / threads=9 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=3 / b_pyramid=0 / b_adapt=1 / b_bias=0 / direct=1 / weightb=0 / open_gop=1 / weightp=2 / keyint=250 / keyint_min=25 / scenecut=40 / intra_refresh=0 / rc_lookahead=40 / rc=crf / mbtree=1 / crf=22,0 / qcomp=0,60 / qpmin=0 / qpmax=69 / qpstep=4 / ip_ratio=1,25 / aq=1:1,00
Default : Yes
Forced : No

Audio
ID : 2
Format : AC-3
Format/Info : Audio Coding 3
Mode extension : CM (complete main)
Format settings, Endianness : Big
Codec ID : A_AC3
Duration : 56s 620ms
Bit rate mode : Constant
Bit rate : 448 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Bit depth : 16 bits
Compression mode : Lossy
Stream size : 3.02 MiB (9%)
Default : Yes
Forced : No

So the filesize is now 32.3 MiB vs. 45.6 MiB and the Overall bit rate is 4789 Kbps vs. 6755 Kbps. I think it stills looks fine.

Any improvements are welcome.