No, step down rings do not cause vignetting. Vignetting is a type of optical aberration that occurs when light is unable to reach the image sensor or film in the camera due to obstruction from the lens or other elements in the optical path. This can happen if the lens is not properly aligned with the sensor, if there is a blockage in the optical path, or if the aperture is too small for the amount of light being transmitted. Step down rings simply attach to the camera's lens and allow you to use filters or other accessories with a smaller diameter than the lens itself. They do not obstruct the light path in any way and therefore cannot cause vignetting.