From the web:
The Heliopan 27mm Close-Up Lens is a convex diopter designed to decrease the minimum focus distance of any lens in order to produce a shorter working distance for close-up photography. Available in varying strengths, close-up lenses also feature 27mm front filter threads for combining two or more filters to fine-tune results.
The front and rear surfaces of the filter both feature a single anti-reflection coating. Each coating helps to increase light transmission by reducing reflections, scattered light, and ghosting. Additionally, they offer protection to the filter substrate while helping to keep the glass clean for longer than uncoated glass.
This Heliopan filter is constructed from Schott glass for increased optical clarity and color fidelity. It features a brass filter ring for durability as well as jamming prevention.
Convex lens design and +3 dioptric strength lessens the minimum focusing distance of any lens for close-up shooting.
Single coatings on both the front and rear offer protection to the glass and help to improve light transmission and image quality.
Constructed from Schott glass for optical clarity.
Brass filter ring helps to prevent jamming and provides additional strength.
Dual threading with 27mm front filter threads permits combining with other filters.