Bifocal glasses
Single vision glasses, having the same power in the entire surface of the lens, are meant for either distance, or near, or for constant use…as in children and youngsters.
Bifocal glasses, with two different zones, have distance power in one zone and near power in the other (usually lower) zone.
Bifocal glass types may be either Kryptok or D-bifocal. Kryptok lenses incorporate the near segment in the shape of half moon, and D-bifocal, as the name suggests, have the near segment in the shape of a `D’; and these are also known as straight-top bifocals…their usage being in older adults usually over 40 years of age.
And, for those who resent the dividing line between the two power zones in a bifocal lens (especially for women), progressive power lenses are available with no dividing line and with an additional progressive zone for intermediate distances.
Picture shows a progressive power lens from my book "Ophthalmic Dispensing Optics".
Dr. Narendra Kumar
OphthaCare Eye Centre
C2C/236A Janakpuri, ND58 more