What does the q-scale option on the magic lantern firmware do exactly?

[cherlique]

i know it is used to adjust the bit-rate, but wil the increase of image quality be visibly noticeable?

[drapeama]

From the ML Wiki page:

QScale (VBR)

This mode is constant quality, variable bitrate (VBR). You can set QScale value from -16 to +16, and lower numbers will result in higher bitrates.
The exact bitrate varies a lot with frame complexity, and there is no direct relationship between QScale and the actual bitrate.
You will see a birate meter near the recording dot (550D/60D/500D). Use it to see the actual bitrate, which is displayed as:

• instantaneous (for last second)
• average (for the entire movie)

Note that unexpected changes in frame complexity may increase the bitrate to very high values, and the camera will stop recording.
In many cases, QScale settings around -8 will give results close to the default bitrate.

[mkkid28]

I uploaded a PSD that compares different bit rates above one another - http://mattthomasfilm.blogspot.com/2...ing-magic.html

Basically what I could see there was less artefacts (less blocky) with higher bit rates, but I don't really think the trade off was worth the increase in size and an increased chance of the recording stopping automatically due to buffer filling up.