I have owned a Canon 300 f4 L IS for about a year without much use until now. I use is with a 35 mm camera, 30v. I have just noticed that all shots with this lens have a slight overexposure along the bottom of the frame in a wedge shape being larger in the right hand corner. Has anyone had this experience? What could cause this? It happens with and without a 1.4 extender and with all film types.