The often ignored calculation of vertex distance (VD). The problem is that vertex distance-that is, the distance, measured in millimeters, between the patient's cornea and the back of the ophthalmic lens-has an effect on the perceived power of the lens. It's even more important in higher power lenses:+/-5.00D or more.
Progressive lenses or progressive addition lenses (PALs) are the most popular multifocal lenses today, offering an effective and great looking vision solution for individuals with presbyopia (age-related near vision loss). As we age, particularly after the age of 40, our near vision begins to deteriorate.