Question

PD measuring instruments: A well-designed PD measuring instrument should rest ______ the bridge of the subject's nose exactly as a frame would. This most accurately approximates the way the glasses will position themselves. It should also position the measuring plane at the approximate spectacle plane.

A) against;
B) above;
C) below;
D) behind;

Answer 1

A PD measuring instrument should rest against the nose for accurate measurements, with the measuring plane at the spectacle plane. For an object 25 cm from the eyes, the spectacle lens must create an image 1 m away, resulting in an image distance of -98.5 cm and an object distance of 23.5 cm from the lens. The correct option is a.

Step-by-step explanation:

A well-designed PD (Pupillary Distance) measuring instrument should rest against the bridge of the subject's nose exactly as a frame would. This practice ensures that the measurements are most accurate and truly reflective of how the glasses will sit on the subject's face.

Additionally, the measuring plane of the instrument should be positioned at the approximate spectacle plane to further ensure precise measurements. When an object is held 25.0 cm from the person's eyes, the spectacle lens, which is typically 1.5 cm from the eye, must produce an image 1.00 m away, equating to an image distance (di) of -98.5 cm.

This negative value indicates that the image distance is on the same side of the spectacle as the object. Accordingly, the object distance (do) is 23.5 cm, calculated by subtracting the 1.5 cm distance of the lens from the eye from the 25.0 cm distance of the object from the eyes. The correct option is a.

Answer 2

A PD measuring instrument should rest against the bridge of the subject's nose, simulating the correct positioning of glasses. This ensures the most accurate pupillary distance measurement.

Step-by-step explanation:

The question asks which position a PD (Pupillary Distance) measuring instrument should occupy on the subject's nose to simulate the correct positioning of glasses. The correct answer is A) against. A PD measuring instrument should rest against the bridge of the subject's nose exactly as a frame would. This ensures the PD is measured at the correct point, which most accurately approximates the way the glasses will sit and how they will affect the wearer's vision.

Additionally, in optics and eyewear prescriptions, the spectacle plane is important when considering the distance from the lens to the eye, represented as di (image distance), and do (object distance). A key concept here is to understand that for clear vision, an eyeglass lens must produce an image on the retina at a distance suitable for the viewer's needs, whether for close-up or distant vision.