Hollis M1 Frameless Scuba Dive MaskVisibility is the most important aspect of any dive and the M 1 Mask raises the bar for optical quality and distortion free vision. The lens is ultra clear glass which is valued for its attractive appearance and optical qualities. The low iron content produces a higher light transmittance and a reduced green tint inherent in other glasses. This makes the lens distortion free and helps improve color and visibility in low light conditions underwater.