An image that freezes action may be very interesting and impressive, but it is also possible to introduce a more dynamic flowing feeling into an image by allowing the subject, or some part of the subject, to blur in a controlled manner. The use of a slow shutter speed, or a long exposure, is the most obvious way to introduce motion blur in to an image.The key consideration is the selection of an appropriate shutter speed. If the subject is moving very slowly, as in the case of a train pulling out of a station, the shutter speed must be slow enough for the desired amount of movement to take place during the period when the film or sensor is exposed. However, if the subject is moving very rapidly, as in the case of a low-flying jet aircraft at an air display, a much faster speed should be set to prevent the subject moving right across the frame during the exposure period.
In the case of simple linear motion across the frame, a suitable shutter speed is not too difficult to calculate. Digital cameras also allow photographers to see their results immediately and hence make suitable adjustments for further shots. However, the situation gets a little more complicated when the primary motion of the subject is not linear. For instance, a photographer might try to capture a child moving in circles on a fairground ride. In a case such as this, blurring will be greater when the child is moving across the frame and less pronounced when the motion is towards or away from the camera. The angular motion is nevertheless constant.
In other cases, such as a racing cyclist moving along a track, some parts of the rider's body will move much faster than others and hence display greater blurring due to motion. The feet and legs will blur more than the torso or bicycle frame, and this characteristic can be used creatively to produce almost sharp images which nevertheless reveal rapid motion through blurring of the legs. Selection of an appropriate shutter speed in such a case is much more critical. The speed selected will depend not only on the linear speed of the cyclist but also on the photographer's distance from the bicycle and the rate at which the pedals are revolving. Start with a moderate shutter speed and examine the results to determine whether the speed needs to be faster or slower. If the whole subject is blurred, try increasing the shutter speed until the bicycle is almost sharp. Then examine the image of the legs to see whether a suitable degree of motion blurring is present. If not, it may be necessary to persuade the cyclist to use a lower gear!
In this sort of situation the camera is probably best switched in to shutter-priority mode. The aperture selected will then be very small (f/16 etc) to compensate for the long exposure period. In some cases, there may be too much light to allow for the desired setting, for instance where an aperture of f/64 is needed but is not available on a particular lens. Two basic options are then available. The first, with a digital camera, is to select the slowest ISO rating available. Unfortunately, digital cameras rarely feature ISO ratings less than 200 or 100, whereas film users can obtain slow film with ratings such as ISO 64, 50 or even 25. A further option is to attach a suitable neutral density filter to the lens. These filters merely attenuate the light reaching the film or sensor without affecting the colour of the image. They are generally available in x2 (1-stop), x4 (2-stops) and x3 (3-stop) varieties.