From Cokin's A series
The Cokin Split Field filter allows you to focus very close to a subject and retain the same clarity for the background. This is achieved by placing a diopter (close-up lens) in one half of the lens frame. The other half has no lens so you can focus on the background object as normal while the diopter focuses on the close-up subject. By turning the filter you determine what is close and what is background.