As Plake points out, the Zebra lenses are awesome. They dont seem to get super dark, but Ive used them above 19k' on glacier and they've worked fine. Definitely not as dark as they standard cat 4 lens (Spectron 4) from julbo. As far as the Bivouak goes, it's a great fully featured glacer glass, but the lense wasnt quite tall enough and the nose bridge not quite wide enough for my face. I had too much of a gap between the bottom of the lens and the bottom of my field of view. The julbo RUN is the best fitting julbo FOR ME. Everyone's face is different YMMV.