Old School Techniques - Split-Field Diopter

In a field mostly dominated by digital technologies, it's nice to share a technique rooted in old-school camera optics. And the fact that this technology can be very affordable is so much the better.

Controlling the field of focus is one way cinematography contributes to effective story-telling. Our eyes naturally travel to the parts of a the frame that are sharp. However, lenses often limit the field of focus in the frame.  A split-field diopter provides the ability to capture two distinct focal planes within the same frame and are often used to maximize deep focus.

Here's Richard H. Kline, A.S.C. discussing the use of split-field diopters on Star Trek: The Motion Picture in Return To Tomorrow: The Filming of Star Trek: The Motion Picture:

We were operating at only 20 foot candles which is very low-it's really hard to get a good depth of field. So, since depth of field was one of the things Bob Wise (the director) wanted to stress, I employed split diopters on most of the scenes. A split diopter is an optic, the equivalent  to bifocal glasses, where the eye sees, looking straight forward, one specific distance, and, when you move to a different viewpoint, sees another increment of distance. The split might be top to bottom or side to side; the diopters are half-moon-shaped, and they cover one half of a third of the lens. They attach right onto the lens, and they can be shifted right and left, and up and down, or whatever...However, with split diopters, you can also pan the camera all you want, you don't have to keep it locked in one shot at a time. All you have to do is slide the split diopters on and off for each focal point, being careful, when you set up the shot, to prepare it so that you've got a shadow, a vertical set element or a hot spot to cover the blend line.

Not a great film in many respects, but Star Trek: The Motion Picture
is chock full of split-field diopter shots.

A number of directors have made extensive use of split-field diopters. The list includes De Palma, Speilberg and Quentin Tarrantino.

There's some debate over how Director of Photography, Gregg Toland achieved deep focus in Citizen Kane. To me it seems more likely that his ground-breaking shots were created using split screen mattes, seamlessly combining live action film (sometimes double-exposed) with matte paintings. 

In any case, cheap split-field diopters are widely available.  (Here's one from B&H.)  This video from PBS|digital studios Shanks FX provides some DIY info along with other Hollywood examples.