The preview is quite coarse, regardless of how the Render Mesh settings are set.
This is with FilletSrf, it’s the same with BlendSrf or similar commands.
Q:
Is there a setting which leads to a finer preview?
Go to the detailed settings and play with the Maximum Angle. It’s set to something like 20 by default for the “Smooth and Slower” preset and that may be too much for your case if you want nicer preview.
Note that this is only affecting how it is rendered by the GPU, the underlying NURBS surface is still mathematically perfect and the Render Mesh settings have no effect on further operations. Eg. if you extract isocurves, they will be perfectly smooth regardless.
I know about the mesh settings, but they have no influence on the preview.
That’s why I asked.
Oh, sorry, I didn’t catch that. I guess it’s done for performance reasons so that when you play with the sliders and it needs to constantly recalculate the mesh, it would be responsive. I don’t think you can change this. Does it matter that much?