The diopters from Schneider Optics are available in Full, Split, and Spot options. The 4.5" is a Round Drop-In Filter.
Full
A classic full diopter reduces the close focus distance and even allows you to take macro images without using a dedicated macro lens.
Split-Field
One of the most unique cinematography tools, the split-field diopter creates two depths to your frame, allowing you to keep two subjects in focus at the same time.