Final answer:
Parallactic errors in using a PD ruler increase when the dispenser is too far from the subject, ideally beyond 40 cm. This concept is connected to the way images are perceived through lenses and how errors in focal length affect vision correction.
Step-by-step explanation:
The question you're asking is about understanding why parallactic errors occur when using a PD (pupillary distance) ruler. Parallactic error can be increased if the dispenser is too far away from the subject, which should ideally be close to normal 40 cm (16 inches) for accuracy. When the distance is greater than this standard, the likelihood of an increased error arises. This relates to how images are perceived through lenses, where a longer focal distance can affect image location and size perception.
In correcting vision issues such as farsightedness, an image appearing farther away is leveraged by using a converging spectacle lens, which increases the eyes' converging power when the focal length is too long. Conversely, a diverging lens is used for myopia (nearsightedness) to offset an eye that overconverges light rays due to a too-short focal length. Hence, understanding the relationship between object distance, lens power, and perceived image distance is crucial for accurate PD measurement and for correcting vision issues with the appropriate lenses.