Open Box Hollis Mask M-1Visibility 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 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 underwater.