As photographers, there are times when we have to interpret the scene as the camera will record it and not as we see it with our eyes. This can range form the subtle changes in the colour of light to allowing movement to be recorded as a blur. Sometimes this may not be as easy as it first may seem. It is certainly not something you will get right first time (unless you are lucky). Pre-visualisation is key – you need to analyse the various elements in the scene and how they will be recorded using the various settings on the camera. This can range from how much of the scene you want in sharp focus (depth of field) to the setting of the correct shutter speed so that it depicts the right amount of motion as you intend to record it.
If we use water, or more importantly flowing water as an example. Do you want to record it motionless (frozen (in photography terms) where the water droplets are suspended in the air or create the milky smooth effect of a fast flowing river? The choice is entirely up to you and is within in your control.
As you are aware, to freeze the action depends on setting a fast enough shutter speed according to the amount and speed of movement in your scene. Using flowing water as an example, to freeze the motion of water cascading over rocks may need a shutter speed in excess of 1/1000th of a second.
While to create a smooth milky effect of a fast flowing river may require a shutter speed as slow as 15 seconds.
How fast or slow a shutter speed is for to decide. Start with what you think is the correct speed, take the shot and review the results on the back of the camera. As you are varying one of the 3 elements of exposure, you will need to change the Aperture or ISO setting to balance the exposure.
If you are using shutter speeds below 1/30th of a second you will need to support the camera on a tripod to avoid any unintended motion blur being recorded by camera shake.
This shot was then using the following settings ISO – 100 Aperture – f8.0, Shutter – 8 seconds