Panning

This technique involves using a fairly slow shutter e.g. 1/30s. The photographer tracks the subject in the viewfinder as it approaches moving the camera in an arc that follows the movement of the subject. When the subject is in front of the photographer the shutter button is pressed. The photographer "follows through" after the shutter is released to keep the movement smooth.

The task is made a little harder because the viewfinder of an SLR goes black when the mirror swings up during exposure.

It can take some practice to get consistent results

Shutter Speeds

The shutter inside a camera (in conjunction with the aperture) regulates the amount of light that reaches the sensor (or film).

Shutter speeds on most SLRs range from an incredibly short (fast) 1/4000s to as much as 30 seconds.

Fast shutter speeds will freeze motion. Just how fast a speed is required to freeze the subject varies with the velocity of the subject, its distance from the camera and the direction of the movement.

If a racing car is travelling at 200kph it obviously needs a faster shutter speed to freeze it than if it was travelling at 50kph.

If the car is a long distance from the photographer the relative movement across the frame is naturally much less than if it was just a few feet away travelling at the same speed.

If the car is moving towards or away from the photographer there is much less relative movement than if it is travelling across the field of view.

Of course the photographer does not always want to freeze motion. A certain mount of blur can add much to an image. Imagine the racing car shot at 1/8000s it would be very sharp but the viewer wouldn't know if it was moving at high speed or if it was stationary.
To capture the feeling of movement photographers often choose to pan with the subject to produce a far more dynamic image (see below).

Choice of shutter speed is important in all sorts of photography. A good example would be photographing a waterfall. A fast shutter speed will freeze individual droplets in mid-air. It can be a dramatic effect that really shows the power of the waterfall.

A very different result is achieved by setting a long shutter speed e.g. 1/8s. The photographer will need to keep the camera perfectly still (preferably on a rigid tripod) the end result being sharply defined rocks with the water a silky blur.

Another factor that affects choice of shutter speed is the focal length of the lens being used. A long focal length e.g. 300mm will magnify the subject but it will also magnify any slight movement in the photographer's hand. This "camera shake" will cause the image to be blurred unless a faster shutter is used. Shorter focal lengths (wide angle lenses) can be used at much slower shutter speeds as long as the subject itself is not moving.

Image Stabilisation (IS) built in to some lenses or cameras can significantly reduce the effect of camera shake. For example if you can keep the camera steady with a "normal" lens at say 1/125s, you may be able to go 2-3 stops slower i.e. 1/30 or 1/15 of a second with IS.
IS does not of course prevent blur if the subject itself is moving.

Because fast shutter speeds allow less light to reach the camera's sensor they require more light and/or wider apertures to compensate. In low light you will very soon reach the limit of the camera's aperture range. You can increase the camera's ISO setting to allow higher shutter speeds but very high ISOs produce a lot of optical "noise" and reduce image quality. In very low light you will only be able to use slow shutter speeds.

Very slow shutter speeds in bright light (e.g. when trying to blur a sunlit stream) can be a problem as once the the lens has been stopped down to it's minimum aperture, and the ISO set to its lowest setting there could still be overexposure.
Neutral Density (ND) filters can be used to further reduce the amount of light reaching the sensor and so allow slower speeds.

In Shutter Priority Mode ('Tv' i.e. 'time value' on Canon, 'S' mode on Nikon) the photographer selects the Shutter Speed to control the way motion or movement is depicted in an image the camera then automatically sets the aperture to give "correct" exposure.

It might be tempting to put the camera in to the "Running man" or sports mode. However this is far less flexible than shutter priority because you do not always want to freeze a subject. Also many cameras will not allow the photographer to shoot RAW files in the more automatic modes.

Shutter speeds can of course also be set in Manual Mode. In which case you set the speed then meter the subject and manually adjust the aperture.