Big circle appears in glass reflections?

[camfran]

I'm having a problem with my glass, I get this big sharp edge between reflection and non-reflection. The shape of the edge is part of a circle. I don't think it's to do with the Fresnel falloff, as the boundary is there whether the falloff is On or Off.

 

The only thing which seems to change the location of it is the IOR, and of course moving the camera viewpoint. In the images, the circle is a vertical circle line, should be quite obvious.

 

The material is a VrayMat, I'm using a refraction map to get the Reglit glass effect.

 

Does anyone have any ideas??

 

Thanks very much