background gradient

hi everybody
Im asking you a beginer question .
no matter how I render, 8bit, 16 or 32bit in cinema 4d r13 , i cant get the background gradient to look ok
and look like in this image ( cant upload the image because Im not level 10 :(( )
but probably you know what im mean about the gradient is not smooth and make some circles

can anyone help me understand where im wrong ?
thank you very much