The game looks much better with activated AA, unfortunately the functionality is experimental and it causes numerous errors in the presentation. The rest does not differ so much from the lower settings. In my view.

To the question: A good question!

If you click the checkbox to activate the Soft Corona option it will actually shut down the Soft Corona lighting effect.  I know it seems confusing.

What does it do?

It removes the hazy effects surrounding all the in-game models and reduces the "...diffusion of certain types of lighting". It is a personal preference setting as it does not make the graphics inherently better or worse just "different".  This was a setting in Van Helsing also.