Motion Blur is a technique intended to capture movement, instead of having it frozen. The most common way to understand what a motion blur picture is, is to think of a vehicle moving and impressing the sensor in a long exposure photo, and recording a blurred trail.
This technique can be used to blur objects in the background leaving the foreground sharp, or blurring the foreground against a sharp background. According to the position of the camera different effects can be achieved. It can be fixed on a tripod to capture the movement in the surroundings. In still scenery, it can be the camera which moves, anchored to an object like a skateboard, a car, a bicycle; in this case the “vehicle” will be sharp and the surrounding blurred, giving an impression of movement. Or the camera can move following an object and blurring the background: this last technique is more usually called Panning and should be treated in a separate topic. Also Light Painting, discussed already in a previous post, is a particular case of the motion blur technique.
The concept behind this technique is simple to understand and quick to put into practice, even if a certain amount of trial and error will be needed for fine tuning. First of all we have to slow down the shutter speed, using Manual mode (M) or Shutter Priority (Sv). It’s difficult to say how long should it last, since this is dependent on the velocity of the subject.To obtain a blurred effect we need a slower speed for slower objects or objects further away from the subject. The closer the object, or more rapid, and hence faster the speed of the shot (but still slow) . In most cases we will need a stable tripod or surface to take the shot.
Usually during a night time shot, there aren’t many issues involved in taking a long exposure photo, however this isn’t the case for a daylight shot. In fact, slowing down the shutter speed too far in a daylight shot often results in an overexposed picture. A solution to this would be to try to close as much of the aperture as possible, using Manual mode (M) or Aperture priority (Av), and keeping ISO at the minimum value. If it is still not enough, consider using a Neutral Density filter.