ND filters are a great tool for a variety of photographic situations, and we'll look at what the filter is and where it can be used here.
What is an ND Filter?
Basically, an ND filter restricts the amount of light that can get through to the camera's sensor, allowing you to shoot at slower shutter speeds without overexposing the shot. ND filters are plain grey, are available in different strengths and they don't change the colours in your shot.
Where will an ND filter be useful?
Waterfalls - When photographing waterfalls, a popular 'look' is the silk-like water image that sees water movement given an almost mist-like quality. To achieve this, you need to set your camera on a longer exposure and by using an ND filter, you'll be able to extend this exposure time without bright spots appearing or worse case scenario, over exposure ruining your image totally.
Busy City Shots - If you want to photograph a famous landmark, or simply want to shoot buildings without people in your shots you usually have two options: One, either wait until it's later on and there aren't as many shoppers and tourists or two, get up really early to miss the crowds. However, with the right shutter speed, something which can be achieved with the help of an ND filter, you'll able to erase moving subjects from your shot, as the shutter speed will be so long that they won't show up on the shot as they pass by. An exposure of around 4 seconds should be enough to make this work effectively.
Aperture - If you are shooting in bright conditions you may find the aperture the camera is selecting is small and the resulting picture will have far too much front-to-back sharpness, known as depth-of-field. This is often the case with portraiture or flower photography where a distracting background ruins the photo. Using an ND filter will help you open up the lens and provide a shallow depth-of-field.
Flash - Another use for an ND filter would be when using flash. You can often reduce the exposure of the flash using auto settings, but for close-ups that may not be possible. The ND filter will provide the key to this essential barrier.