I am using a 20D with the 17 - 85 mm lens. After using it for wedding pictures last weekend I notice distortion of the steps and windows inside the church. (the ends curve downward) I have not noticed this happening on any other shots, just ones with strait lines running horizontally across the frame. (The shots were taken with the couple in the center looking down the middle of the aisle.)
Any advice on how to avoid/fix this problem?