Threaded magnifying elements for close focus — boost magnification without swapping glass. Cheaper than macro lenses, but edge softness at wide apertures.
You screw a supplementary lens onto your standard lens and suddenly you're focusing on centimeters instead of meters — that's the core idea, and it works significantly cheaper than a true macro lens. The supplementary lens sits like a filter between the front element and the subject, but it works optically actively: it shortens the effective focal length of the system or, in other words, it brings the focus point closer without you having to change the lens itself.
In practice, you always need supplementary lenses when a close-up is spontaneously required and your set lens doesn't cover it. A 50mm prime lens becomes a detail camera with a +2 or +4 diopter lens — ideal for macros of rings, jewelry, or fine texture details. The strength is indicated in diopters: the higher the number, the closer the minimum focus distance. On set, you often work with sets of two or three different strengths to remain flexible. The big advantage: weight, handling, and you don't need mechanical focus adjustment on the lens, which saves time during changes.
The disadvantages are real and always appear at the edges of the image: vignetting, chromatic aberrations, and loss of sharpness. A good supplementary lens minimizes this, but it's never completely gone. Therefore, you place your subject in the central frame where the optical quality is highest. With longer focal lengths (85mm+), the problems are more pronounced than with shorter ones. High-quality manufacturers offer multi-element supplementary systems that are better corrected — they cost more, but the image holds up longer. For budget productions, you simply screw on the cheap one and adjust your framing accordingly.
A practical tip from everyday set life: test supplementary lenses ON the camera you will be using BEFORE shooting. Every lens reacts differently. And if your production uses multiple cameras, make sure you have the same supplementary lenses on all of them — nothing is more annoying than focus inconsistency between parallel cameras during an action scene. Supplementary lenses are not a high-end tool, but they solve real problems in a set-friendly way.