The split-field diopter is a practical lens attachment that can add an unusual and visually exciting effect to your movie. Here’s what you need to know.Top Image: Carrie via MGMWhat Is It?A split-field diopter is a partial lens that is attached to the front of your camera’s lens. Essentially, it is a filter that will directly alter the focus within your shot. It’s a partial lens because it only has glass covering half of the lens space.Split-Field Diopter via HoyaWhat Does It Do?The split-field diopter allows you to focus on a subject (or object) positioned close to the camera on one side of the frame while also focusing on a more distant subject positioned on the other side of the frame. Instead of having to rely on a rack focus between a nearby subject and a distant subject, both close and distant focal planes will be sharply focused at the same time. Here’s an example of what it can do, via biscuitsalive on YouTube:Since the focus will be set both near and far, a visible blur can be seen around the foreground subject, who is positioned closer to the lens. This blur occurs because the near and far focal points each have their own separate depth of field. Because of the design of the split-field diopter, and the jump between the foreground DOF and the background DOF, the blur will be cut off near the center of the frame, at which point the background subject will be in sharp focus.Who Has Used the Split-Field Diopter?Having used the split-field diopter in most of his films over the years, Brian De Palma is perhaps the most famous practitioner, as seen in this video from Vashi Nedomansky:Director Robert Wise and his cinematographer Richard H. Kline used many split-field diopter shots in Star Trek: The Motion Picture. According to a well-regarded cinematographer, David Mullen, Star Trek: The Motion Picture may hold the record number of split-field diopter shots in a single film, with as much of 60% of the movie making use of this interesting lens attachment.Star Trek: The Motion Picture via ParamountThe split-field diopter shot was used more heavily in the seventies and eighties, but modern filmmakers still employ the technique. Quentin Tarantino has been known to use a split-field diopter shot from time to time.Reservoir Dogs via MiramaxAnd countless others have made use of the split-field diopter:Equilibrium via DimensionBride of Chucky via UniversalUnforgiven via WarnerHow Do You Get One?The marketplace isn’t overflowing with split-field diopters, but they are available. B&H has a Cokin A 111 +1 Filter for about $30. Match that with a Cokin “A” series filter holder (with a ring that matches the diameter of your main lens) for about $20, and you are in business.How Do You Use It?The distinctive separation between the foreground focal plane and the background focal plane is something that cinematographers have typically worked to conceal in some way or another. In order to achieve this, the shot that makes use of a split-field diopter should ideally be static. This means that you should avoid pans, tilts, dollies, or handheld shots while using the split-field diopter. That is, unless you want to emphasize the focal disparity in your image.The other consideration is the blocking of your subjects, which must be done very carefully and precisely so that focus is maintained. You cannot position subjects or important visual elements in between the near and far focal planes unless you want these elements to be out of focus. The split-field diopter will test your composition abilities within a static frame and also how well you can stage tableau blocking.Why Would You Use It?All the President’s Men via WarnerThe split-field diopter creates a depth of focus effect in your image that is visually dynamic and a little unsettling. In an emotionally charged medium such as film and video, this means that you can use a split-field diopter shot to convey important story information about a character’s internal state or draw attention to an important foreground character or detail that a background character may not have noticed.What Is the Effect?Casualties of War via ColumbiaThe layered focal planes give the sense of a traditional split screen effect except that with a split-field diopter, your division of the frame becomes an in-camera practical effect. This creates a psychological separation between the subjects who inhabit both focal planes. In conveying underlying story information, the shot can suggest unsettled or separated individuals. When everything is said and done, the meaning of a split-field diopter shot is up to you and will depend entirely on the story that you want to tell. It’s a nice accessory to have up your sleeve.Star Trek: The Motion Picture via ParamountFor further reading, check out Notes on the Split-Field Diopter by Dr. Paul Ramaeker.What are your favorite uses of the split-field diopter? Share your thoughts in the comments below.