I was lighting white seamless last night with two 400W strobes and wide angle reflectors. Just for fun I turned on highlight alert to see where it was blown out and where it wasn't. However, there was one section on the right hand side of the seamless that always seemed to prevent the blinkies. I even turned up the strobes and still it remained, and you could see the fall off gradient. So I'm not sure why but I switched to landscape orientation and boom the entire top half of the seamless registered black to indicate it was over exposed. Again went back to portrait and yup the right side feathered off again. Here is a rough example below, the black section being the highlight alert. If anyone can explain why I will be hugely grateful! Camera is 5D MkII with Sigma 35mm 1.4 shooting at f/11.