The SRB 1.2 Full Neutral Density Pro Glass Filter is a high quality 84mm P Size Neutral Density glass filter that reduces light entering the camera lens by 1.2 (4 Stop), allowing the camera to perform with a higher aperture to produce a result much closer to how it appears to the eye.
The purpose of a Neutral Density filter is to allow you to have more control over the exposure of an image. The ND filter stops light reaching the camera sensor by a specified stop, allowing you to use a higher aperture for a longer amount of time.
For best results, use an exposure guide to help you set up and focus your shot without the ND filter. Then, when you're ready, add the ND filter and start shooting.