Generally, diopters are lens attachments that shorten the close focus distance of lenses by reducing the focal length of the optical system. Their strength is given as reciprocal of focal length in meter (so called diopter), e.g.: + 1 diopter = 1 meter infinity focus plane + 2 diopter = 0.5 meter infinity focus plane
Classic full diopters reduce the close focus distance and even allow you to take macro images without using a dedicated macro lens.
A split diopter is a standard diopter element which is precision-cut in half and mounted in a ring. It allows you to focus both fore- and background at the same time. Hence, it can attract the viewer's attention to an object in the back while the main scene remains focused.
Basically, a spot diopter is a classic diopter with a plain, polished area in its center. This leads to a sharp image in the middle and an out-of-focus area towards the edges. For dreamy images with an in-focus center.
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Close-up lens attachment that shorten close-focusing distance of lenses by shortening the focal length. The higher the strength the shorter the focal length and the shorter the focusing distance. The strength is given as reciprocal of focal length in meter. A Split diopter is a standard diopter element which is precision-cut in half and mounted in a ring. It creates a shot with two planes of focus rather than one. Makes a sharp image of the foreground (captured through the diopter) and the background (captured through the uncovered half of your camera’s lens).