Final answer:
ST bifocals, also known as segmented trifocals or simply trifocals, have three regions of focus: distant, intermediate, and near vision. They were developed to cater to mid-range vision needs along with the traditional near and far sight correction in bifocals.
Step-by-step explanation:
ST bifocals are commonly known as segmented trifocals or sometimes just trifocals. This alternate term emphasizes that they have three regions of focus: one for distance viewing, one for intermediate distances, and one for close-up tasks, such as reading.
Historically, bifocals were developed to provide two levels of vision correction, typically for distance and near-sight necessities. The introduction of an intermediate section created trifocals to additionally cater to the mid-range distances that are commonly encountered in activities like computer use. Bifocals generally have a distinct line separating the different lens powers, whereas trifocals include this intermediate band.
It’s essential to be aware that even though the term ‘ST bifocals’ may suggest only two focal points, in this case, ST stands for “segmented trifocals,” indicating the presence of three distinct viewing areas. Thus, when choosing corrective lenses, understanding the different types that are available and the specific needs they address is key to ensuring optimal vision support.