What's the difference between desaturation and Black and White adjustment

[dmstraker]

What is the technical difference between desaturating with HSL adjustment and using the Black and White adjustment?

An example: Take Monet's 'Sunset in Venice':

Now turn down the saturation to zero.

This demonstrates a neat trick where the sun and background are the same luminosity, so desaturating colours makes the sun disappear.

Now make it monochrome with the Black and White adjustment. The sun is now clearly still visible. So what is happening here that is different?

[- S -]

I made a post at the following link, which has some screenshots (with RGB spectrum) of the differences between different methods:

https://forum.affinity.serif.com/index.php?/topic/36453-can-you-convert-from-color-to-grayscale-andor-blackwhite-in-affinity-designer-andor-photo/