Q:

Loss of quality while adding subtitle via FFmpeg

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.

A:

As you are hardcoding subtitles, the video (with the subtitles added) will be re-encoded. 
You can use the CRF rate control method to modulate the quality of the output.
So, start with
ffmpeg -i grdedFinal.mov -vf subtitles=portSbs.srt -crf 18 -c:a copy gradedFinalwithSubs.mov

If the quality's not acceptable, lower that value till it is - in exchange for a larger file. Of course, don't decide based on file size but on visual quality. x264 is very good at compressing video while maintaining subjectively perceived quality.

