In aperture priority mode, take a pic with aperture wide open, then, stop down the aperture to about f/8, take a series of pic (about 7-8 bursts), does it change in brightness. If there's no change, then good to go, if 2nd pic is too dark, then aperture isn't closing as it should. In the 7-8 bursts, are there any fluctuation in brightness, if there is, then the aperture blades are sticky.