photographing the sun. This is known as reciprocity failure of the material (film, paper, or sensor). Motion blur Motion blur is caused when either the camera or the subject moves during the exposure. This causes a distinctive streaky appearance to the moving object or the entire picture (in the case of camera shake). Motion blur can be used artistically to create the feeling of speed or motion, as with running water. An example of this is the technique of "panning", where the camera is moved so it follows the subject, which is usually fast moving, such as a car. Done correctly, this will give an image of a clear subject, but the background will have motion blur, giving the feeling of movement. This is one of the more difficult photographic techniques to master, as the movement must be smooth, and at the correct speed. A subject that gets closer or further away from the camera may further cause focusing difficulties. Light trails are another photographic effect where motion blur is used. Photographs of the lines of light visible in long exposure photos of roads at night are one example of the effect. This is caused by the cars moving along the road during the exposure. The same principle is used to create star trail photographs. Generally, motion blur is something that is to be avoided, and this can be done in several different ways. The simplest way is to limit the shutter time so that there is very little movement of the image during the time the shutter is open. At longer focal lengths, the same movement of the camera body will cause more motion of the image, so a shorter shutter time is needed. A commonly cited rule of thumb is that the shutter speed in seconds should be about the reciprocal of the 35 mm equivalent focal length of the lens in millimeters. For example, a 50 mm lens should be used at a minimum speed of 1/50 sec, and a 300 mm lens at 1/300 of a second. This can cause difficulties when used in low light scenarios, since exposure also decreases with shutter time. Motion blur due to subject movement can usually be prevented by using a faster shutter speed. The exact shutter speed will depend on the speed at which the subject is moving. For example, a very fast shutter speed will be needed to