If your time exposures are not sharp, do not blame your camera or lens. Even thought your camera is secured on a tripod you may be suffering from anti shake (image stabilization) blur. This anti-shake feature will definitely help with your hand held exposures of 1/4 second and faster but the opposite is true for the longer ones. It seems there is a floating element inside the lens (Canon) held somewhat steady by a gyro but it will cause the element to vibrate a little which will reduce the sharpness during long exposures. This will be more noticeable for the photos you take at night where there are point light sources in the frame. You will recognize what should be sharp points of light to appear football shaped when you enlarge the image.
For those of you who are using a point and shoot, you may be out of luck if you cannot turn off this feature.
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