How to Change a Lens Properly and Safely
When changing a lens, turn off the camera (although that is not always possible, as you may be scrambling to grab a fleeting shot). Changing lenses with the camera on can cause static electricity to attract dust to the inside of your camera and can cause blotches on your pictures. Have the next lens you are about to use ready and shield the camera body from the elements when the first lens is removed. Practice changing lenses to get used to doing this efficiently and as delicately as possible.