Using a slow shutter to shoot during the day or at maximum aperture may result in white blowout, in which case, a Neutral Density (ND) filter might come in handy. Here, I will share the pros and cons of using an ND filter. (Reported by: Kazuo Nakahara)
An ND filter expands the range of possibilities in expression
An ND filter is a filter mounted in front (or at the back) of the lens to reduce the intensity of the light entering the camera. It is often used when you want to slow down the shutter speed even more during day photography. With this, you can express yourself by making use of blur at a slow shutter speed while preventing white blowout from occurring, e.g. the flow of water, movements of people, and so on. Although you can increase the f-number to slow down the shutter speed, the sense of sharpness of the photo may be lost due to blurring caused by diffraction at a small aperture (i.e. when shooting with a large f-number). By using an ND filter, there is no need to narrow the aperture, and the image quality remains unaffected when shooting with a slow shutter during the day.
In addition, if you try taking photos with a bright prime lens at maximum aperture on a clear day, an ND filter will allow you to take pictures with a bokeh effect even if the scene is over-exposed. However, as the amount of light entering the camera is reduced, cameras with an optical viewfinder will suffer from the drawback of having a darker image seen through the viewfinder.