Tiffen 4x4 2x (0.3) Half Color Neutral Density Glass Filter with Graduated SE (Soft Edge).
Color-Grad® Neutral Density Filters
Often, you want to balance light intensity between two areas within a scene. This is important outdoors to allow more sky detail while properly exposing the foreground. Exposing for the foreground will produce a washed-out, over-exposed sky, exposing for the sky will leave the foreground dark, underexposed.