aberrations (spec)

Question 1

describe the importance of the relationship between paraxial and marginal rays for spherical aberration

Answer 1

For a lens with spherical aberration, the best approximation to use for the focal length is the distance at which the difference between the paraxial and marginal rays is the smallest.

Question 2

outline the basis of paraxial theory (1)

Answer 2

paraxial and monochromatic assumptions allow idealised point focuses

Question 3

what is the best practical case for spherical aberation

Answer 3

F1/F2 = 6

Question 4

define an optical coma (1)

Answer 4

Coma is an aberration which causes rays from an off-axis point of light in the object plane to create a trailing “comet-like” blur directed away from the optic axis

Question 5

define an optical stop (1)

how is it related to aberrations (1)

Answer 5

• something in the optical system that limits the diameter of the pencil (eg. pupil)
• non paraxial rays are a major source of aberrations -> using a stop can improve image quality (a reduced aperture eliminated peripheral rays)

Question 6

define oblique astigmatism

Answer 6

Oblique astigmatism is an aberration of off-axis rays that causes radial and tangential lines in the object plane to focus sharply at different distances in the image space.

Question 7

outline the optimal fitting process of spectacles (3)

Answer 7

• bi-aspheric lenses have one aspheric surface and one atoric surface to optimise the performance of both meridians
• patient has to tilt head back so that lens is perpendicular to the floor to mark pupil center
• > optical centre will drop 1mm for every 2° of tilt

nb: incorrectly fitted lens will result in aberrations due to the eye being off-axis in the optical system