Final answer:
In the case of positive lenses, the near PD needs to be reduced to increase the segment inset of the bifocal, accommodating for the necessary eye convergence at near distances.
Step-by-step explanation:
For positive lenses, the near PD (Pupillary Distance) would need to be reduced, meaning the segment inset of the bifocal would need to be increased. This adjustment is necessary to allow for the convergence of the eyes when looking at near objects through the lower segment of the bifocals. For example, if a lens with a power of +0.45 D is considered, and the power needs to be adjusted to correct for presbyopia (far-sightedness) at a closer working distance (e.g., when reading), reducing the PD would result in the optical centers of the lenses moving inward. This provides a greater degree of convergence support, which is needed for the visual demands at near tasks. Conversely, for myopia (nearsightedness), lenses with negative powers, like -5.00 D, help to produce an image that is smaller and on the same side of the lens as the object, as shown in case 3 images discussed in your materials. The location of these virtual images is important for understanding how lens power and bifocal design work to correct refractive errors in the eye.