Re-encoding video library in x265 (HEVC) with no quality loss

I am trying to convert my video library to HEVC format to gain space. I ran the following command on all of the video files in my library: #!/bin/bash for i in *.mp4; do #Output new files by prepending "X265" to the names avconv -i "$i" -c:v libx265 -c:a copy X265_"$i" done Now, most videos convert fine and the quality is the same as before. However, a few videos which are of very high quality (e.g. one movie print which is of 5GB) loses quality -- the video is all pixelated. I am not sure what to do in this case. Do I need to modify the crf parameter in my command line? Or something else? The thing is, I am doing a bulk conversion. So, I need a method where avconv automatically adjusts whatever parameter needs adjustment, for each video. ### UPDATE-1 I found that crf is the knob I need to adjust. The default CRF is 28. For better quality, I could use something less than 28. For example: avconv -i input.mp4 -c:v libx265 -x265-params crf=23 -c:a copy output.mp4 However, the problem is that for some videos CRF value of 28 is good enough, while for some videos, lower CRF is required. This is something which I have to check manually by converting small sections of the big videos. But in bulk conversion, how would I check each video manually? Is their some way that avconv can adjust CRF according to the input video intelligently? ### UPDATE-2 I found that there is a --lossless option in x265: http://x265.readthedocs.org/en/default/lossless.html . However, I don't know how to use it correctly. I tried using it in the following manner but it yielded opposite results (the video was even more pixelated): avconv -i input.mp4 -c:v libx265 -x265-params lossless -c:a copy output.mp4