vision because the front surface of the optical system is the perfectly spherical lens. Both approaches have advantages and drawbacks. Toric lenses must have the proper orientation to correct for astigmatism, so such lenses must have additional design characteristics to prevent them from rotating out of alignment. This can be done by weighting the bottom of the lens or by using other physical characteristics to rotate the lens back into position, but these mechanisms rarely work perfectly, so some misalignment is common and results in somewhat imperfect correction, and blurring of sight after blinking rotates the lens. Toric soft lenses have all the advantages of soft lenses in general, which are low initial cost, ease of fitting, and minimal adjustment period. Rigid gas permeable lenses usually provide superior optical correction, but have become less popular relative to soft lenses due to higher initial costs, longer initial adjustment period, and more involved fitting. Correction of presbyopia Correction of presbyopia (a need for a reading prescription different from the prescription needed for distance) presents an additional challenge in the fitting of contact lenses. Two main strategies exist: multifocal lenses and monovision. Multifocal contact lenses (e.g. bifocals or progressives) are comparable to spectacles with bifocals or progressive lenses because they have multiple focal points. Multifocal contact lenses are typically designed for constant viewing through the center of the lens, but some designs do incorporate a shift in lens position to view through the reading power (similar to bifocal glasses). Monovision is the use of single-vision lenses (one focal point per lens) to focus an eye (typically the dominant one) for distance vision and the other for near work. The brain then learns to use this setup to see clearly at all distances. A technique called modified monovision uses multifocal lenses and also specializes one eye for distance and the other for near, thus gaining the benefits of both systems. Care is advised for persons with a previous history of strabismus and those with significant phorias, who are at risk of eye misalignment under monovision. Studies have shown no adverse effect to driving performance in adapted monovision contact lens wearers. Alternatively, a person may simply wear reading glasses over their distance contact lenses. Other types of vision correction For those with certain color deficiencies, a red-tinted "X-Chrom" contact lens may be used. Although such a lens does not restore normal color vision, it allows