This is image blur caused by camera movement during the exposure. The longer the exposure (the slower the shutter speed), the more time there is for camera movement to take place. Any movement is also exaggerated with longer focal length lenses (telephotos). There is a simple way to estimate the risk of camera shake – take the effective focal length of the lens and divide it into 1 to get the minimum ‘safe’ shutter speed. So with a 30mm lens, the minimum safe shutter speed would be 1/30sec. However, today’s image stabilisation systems reduce shake and make slower shutter speeds possible.