Fairly simple problem. Whatever object I have modelled once mirrored along the x axis (although the problem occurs along any axis) seems to suddenly go dark. I’m not sure I can explain it any better, but below is an example. On the left we have a simple eye I have made (with SSS on) and on the left the same eye copied and then mirrored along the x-axis. I have only one light in the scene, which incidently is on the right side with the dark eye (note that no matter where I put this light the situation does not change). I have tried this with a number of objects, but the effect remains the same. Also I have copied the eye and placed it in the same position without mirroring it and then the problem does not occur.
Can someone explain to me why this is happening and how I can fix it. Thanks in advance.