Occasionally when shooting sharply defined dark subjects against a bright background (e.g., dark tree branches against a bright sky) I get thin (maybe 2-4 pixels) bright fringes around the dark object. An example is attached, greatly zoomed and tweaked in Photoshop to exaggerate the effect. It was taken with a Canon 50D with a Canon 24-105 F/4L zoom, my most common combo. It was shot at F/11 and 24mm. Questions: How can this be avoided? How can it be corrected in post-processing?