I want to set the video quality through the crf flag and it seems the quality of the encoded video does not change with the setting. It seems that the quality of the video is entirely governed by manually setting the bit_rate. My code snippet looks like

options = {'crf': '51'}
container = av.open(self.output_file, mode='w', options=options)
stream = container.add_stream('libx264', rate=24)
stream.width = 1024
stream.height = 800
stream.pix_fmt = 'yuv444p'
stream.bit_rate = 8000000

Thank you so much in advance for looking at my problem.