Hollis M-1 MaskM 1 Mask Visibility is the most important aspect of any dive and the M1 Mask raises the bar for optical quality and distortion free vision. This lens is an extra 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