Spherical Ametropia 2

Question 1

Blur

Answer 1

Myopia/hypermetropia causes points to be images as blur circles on the retina

Question 2

The Pinhole Disc

Answer 2

• Blur circle diameter is directly proportional to the pupil size and to the ocular refraction, K
• As the amount of spherical ametropia increases, so does the blur circle diameter
• the pinhole disc artificially reduces the pupil size significantly decreasing the blur circle. The value of p is significantly reduced, so decreases the blur circle diameter size

Question 3

The Pinhole Disc and Vision

Answer 3

• will improve vision if blurring is significant
• reduces light level adversely affecting vision
• increases diffraction if vision already good possibly decreasing vision
• if vision improves with pinhole then the indication is that deficit is due to retinal image blur

Question 4

Basic Blurred Image Height

Answer 4

=defined as height of the principal (chief) ray at the retina
- the chief ray is the ray that passes through the centre of the entrance pupil

Question 5

Consequences of Spectacle Magnification

Answer 5

• Spectacle correction of myopia produces minification; objects appear slightly smaller and darker possibly leading to patient preference during subjective refinement
• spectacle correction of hypermetropes makes objects appear slightly larger, including their eyes
• NOT THE CASE FOR CONTACT LENSES