The medium fit mask has a skirt designed for narrow faces. The medium fit mask is identical to the standard model in every other detail. The Atomic Aquatics SubFrame Mask is so durable, it carries a Lifetime warranty against frame breakage. The strength comes from the internal frame (SubFrame) molded directly beneath the surface of the silicone rubber skirt (patent pending) This SubFrame provides incredible strength and rigidity as well as eliminating the bulky external plastic frame in other masks A stainless steel retainer locks the lenses in place and provides added strength to the nose bridge area of the mask A wide field of view and great fit completes the combination of function, style and durability Hydrodynamic styling with no exposed external frame Comes with UltraClear lenses UltraClear is a new and exciting optical quality glass with exceptional clarity and high light transmission, with no color distortion like standard lenses. The exceptionally high light transmittance and lack of distortion in the UltraClear lens maximizes the light available for improved visual acuity, especially underwater in low light conditions Low profile Easy to use squeeze to adjust buckles Adjustable mask strap This medium fit mask has a skirt designed for narrow faces. The medium fit mask is identical to the standard model in every other detail Optional for the SubFrame. Precision ground optical glass lenses are available in diopters between -1.0 to -8.0 in .5 diopter increments (contact us for details)