How close are you and how dark is it? All lenses have a minimum focusing distance. It is very possible that your blurriness is due to movement of yourself or your subject. As the environment gets darker, you camera must shoot with a slower shutter speed and a higher iso. The slower your shutter speed, the greater your chance of motion blur. The higher the iso, the noisier the shot will be. If you are zooming in, the longer focal length makes the depth of field shallower and the chance of motion blur greater.