John - thanks for the moiré info.
Moiré can be caused by a number of things. Sensor resolution is one (not the case here as I'm at 15Mp). Lighting and repetitive patterns (In this case it is a contributory factor). The main reason for Moiré here I think is a combination of the background pattern and the blades on the Iris of the lens. The best lenses will have lots of circular blades and produce an almost circular opening. The Canon L series 70-200 has 8 circular blades. My (relatively) cheap Sigma 150-500 has 9 diaphragm blades so it shouldn't be such a problem. Therefore the main reason must be the patterns caused by the lighting conditions in the background. Its not too often you run up against this one, so I'm not going to worry about it for now.
B.T.W. hold down CTRL and ALT then press the letter to get an acute. e.g. áéíóú
