Artefacts with sun lamp in Cycles


I’m quite new in the use of Cycles, and I’m already facing a problem. In my simple scene, I have a sphere on which I applied a smooth shading. I added a simple sun lamp to illuminate the object and create hard shadows behind the object.

When I render with BI, the result is OK :

But when I use Cycles, the render image displays an awful aliasing separation between lighted area and unlighted one on the sphere :

I tried to increase the size of the lamp (from 5 cm to 30 cm). The problem disappeared, but my shadows are to smooth then :

Can someone explain why it is acting like this and how I can get rid of this mess ? Thx

This is a known limitation - terminator error. To eliminate use Subdivision surface modifier.

OK thanks. That’s what I was afraid of.