What is a characteristic of the basic construction of a progressive addition lens?

The characteristic of a progressive addition lens is that it incorporates a gradual change in optical power from the top of the lens, which is primarily designed for distance vision, to a lower central area where the near vision for close tasks is optimized. This gradual transition, often referred to as a "corridor," allows the wearer to seamlessly shift their focus from distance to intermediate and then to near vision without the visible lines found in traditional bifocal or trifocal lenses.

This design is particularly beneficial for presbyopic patients who require multiple vision corrections and prefer a lens that offers smooth visual transition without abrupt changes. The corridor enables the wearer to adjust their gaze at varying distances easily, accommodating everyday activities like reading or using a computer.

In contrast, the other options describe lens characteristics that do not represent progressive addition lenses accurately. For example, a near portion only or a thick portion above and below a centralized area does not encompass the progressive nature of the lens. Therefore, option C provides a comprehensive description of how these lenses function and their structure, making it the correct answer.