Cows in the Great Plains

A spilt diopter is a very specific type of filter used primarily in cinema as tool get two subjects in focus at the same time even though one is in the background and the other is in the foreground.

In order for the effect to be well defined you have to close down your aperture a little bit and pick a focal length that is wide enough. When you do the opposite and use a longer focal length wide open both areas in focus tend to superimpose, creating this a particular kind of blur and sometimes mimicking the effect of a double exposure.

Following images : red filter + split diopter