The Hoya 27mm HMC NDx4 Video Filter reduces the amount of light entering the lens by 2 f/stops, allowing wider apertures to be selected, which is perfect for portraiture to reduce depth of field.
Subject appears crisp and clear while the background becomes a soft blur.
Alternatively, the ND4 Filter can be used to allow longer shutter exposures than would otherwise be possible and is widely used for subjects such as waterfalls and other nature scenes to blur the moving elements and emphasize the movement.