Originally posted by ckiang
Yes, the sole purpose is to reduce light coming into the camera. But it will not give you a darker picture coz the camera will compensate for it. Why use it? Some scenarios are as follows:
1. You want to get a silky effect when shooting waterfalls
To get a silky waterfall, you will need a slow shutter speed of at least 1/2s or more, depending on the water flow speed. On a bright day, even on ISO 100 and at f/22, you might get a shutter speed of something like 1/8s. With a 3-stop ND filter, you can lower it to 1s.
2. You want to fill flash on a bright day
On a bright day, at your chosen ISO and aperture, you might find that the shutter speed is more than your maximum flash sync speed (e.g. 1/125). Using an ND lets you maintain that chosen aperture and lower the shutter speed to the max flash sync speed and below.
Regards
CK