Minus 2.5 Lens which may be rotated to suit either left or right hand side of the Beaver Discovery mask. Spherical correction ensures a clear & concise appreciation of the underwater world. available from - 1.0 to -7.5 & +1.0 to +4.0 in 0.5 increments. Not suitable for bifocal or cylindrical prescriptions. It is recommended that you seek the advice of a qualified optician.