Photochromic lenses 2-4 are specialphotochromic lenses with colorone basic smoke capable of reaching category 4 in the maximum work phase. You color itone starting point is in colourone very mild smoke that allows for correct facesone also in zone with strong shadow or very low light, and during maximum work it is able to reach category 4 with optimal light and UV conditions, generally used for mountaineering and flying sports to best protect the eye at high altitudes . As with common photochromic lenses, the 2-4 photochromic lens, exposed to UV radiation and sunlight in general, also starts a reactionone reversible chemistry that darkens the surface of the lens until it reaches category 4 (when necessary). Once the irradiation is finishedone of light and UV the lens gradually returns to its original color state. For which sports you can use the 2-4 photochromic lens:  Mountaineering and Hiking: This specialThe photochromic lens was designed with the mountains and the greater intensity of UV radiation and light rays in mind. The idea was to create a dynamic lens for hikers and mountaineers that adapts to every change in light present on their path. Whether you enter a wooded area or a shaded slope, the lens is designed to lighten and darken quickly to allow the athlete constant and perfect vision.one. The starting color of the lens is a light smoke veil which does not oblige the athlete to have to remove the glasses in zone with low light. At the same time, category 2 prevents glare for the eyes when exiting the zone of shade. The achievement of category 4 in the maximum work phase is influenced by the intensity of UV rays and light. It is important to know that this specialand lens is created to reach category 4 only when necessary, thus preventing you from having a faceone too dark of the path. This lens can be used instead of the category 4 lens for extreme expeditions (Everest, K2 and similar) because in terms of protectionone maintains category 4 standards, but at the same time presents itself dynamically to changes in light.  [/ Caption] Paragliding, Hang Gliding and Flying: Due to the same factors described above (more intense UV and light intensity at higher altitudes) the 2-4 photochromic lens can also be used for flying sports which proves to be very effective against the glare of light, and reactive to light changes. It proves to be a very restful lens for the eyes even with very prolonged use, providing maximum relaxation. It is important to specify that this lens is not idonea for use in airplanes (e.g. driving tourist planes or gliders) as the shielding of the glass prevents the reactionone chemistry of the lens because the UV rays are shielded by the glass. In these cases we recommend choosing the category 4 lens. Lens Materials and Feature: The photochromic lenses 2-4 are made of anti-breakage polycarbonaterimto quality. The impact resistance of polycarbonate is fundamental, in fact in the event of an accidental impact the lens does not generate fragments that could damage the eye but distributes the impact across its entire surface forming a spider web effect which invalidates it. The lightness of polycarbonate is also another very important factor in increasing the performance and the comfort of the glasses. Furthermore, the lenses are decentralized and optically perfect, these two characteristics provide the perfect sense of depth and prevent some subjects from feeling "drunk" caused by the use of curved lenses or those with incorrect decentration. CLICK HERE FOR THE COLLECTIONONE COMPLETE WITH SPORTS GLASSES WITH 2-4 PHOTOCHROMIC LENSES