The +4 diopter of this close-up lens are the right power for closer focusing with a 50mm lens (up to approximately 1:3) when you have reached the limit with the NL 2 lens. With 85mm (up to appr. 1:2.1) and 100mm (up to appr. 1:1.9) telephoto lenses, it is the highest power, and it should be used with the main lens stopped down to at least f/8.
Each glass surface is vacuum coated with one layer of Anti-Reflection coating. This single layer helps to prevent internal ghosting and reflections. It also improves light transmission from approximately 92% (uncoated glass) to over 98%.
This filter uses the standard B+W F-Pro filter mount, which has a front accessory thread and is made of brass. Compared to the earlier standard mount introduced in 2001, the F-Pro mount has become thinner. Now it can be used with wide angle lenses, including most 24mm focal lengths on a full frame body, without vignetting. Another advantage of the F-Pro mount is it's modified retaining ring, which is no longer threaded in from the front, but holds the filter glass in place from the back. When removing a filter or lens hood that has been screwed on too tight to the filter, the retaining ring is not at risk of loosening.
NOTE: diopters are mounted in the older mount due to glass thickness.