This study investigated the influence of the light guide shape on the polymerization efficacy of a light-activated resin-based composite cured with LED units as a function of the distance between the tip and the restoration Two different LED units, each with different light guides and shapes, were used. Their ability to cure a single restorative material was investigated. The efficacy of the light-curing system was evaluated by measuring the depth of cure using the Acetone Shake test. Considering the ratio (R) between the entry diameter and the exit diameter of the light guide, the tips with a higher R-value proved to be more efficient if the tip and composite distance (D) was less than 5 mm, while for D>5 mm, the tips with a lower R-value showed better results. The tip geometry of the tested light guide had a significant influence on the depth of cure of the tested resin composites. Therefore, depending on the distance, the more suitable light guide should be selected, based on the clinical situation. An R-value may be a better descriptor for the light guide shape than words such as "normal" or "turbo."