After solving on problem I cam coming to a next one. After adding subtitles, discussed here, with the command:
ffmpeg -i grdedFinal.mov -vf subtitles=portSbs.srt gradedFinalwithSubs.mov
the quality of the generated file significantly decreased. The original file was of size 1,5GB whereas the resulting file was of size ~300MB. The subtitles are added correctly but the compression is unnecessary. In a word I would like for the FFmpeg to add subtitless and do only that leaving the sound and picture quality untouched.