Discussion
Which of the following are off-axis aberration ?
(a) Spherical aberration
(b) Chromatic aberration
(c) Coma
(d) Field curvature
Choose the correct answer from the options given below :
Answer: C
The off-axis aberrations are (c) and (d) only: coma and field curvature.
Off-axis aberrations appear only for object points away from the optical axis and grow with the field angle. Coma makes off-axis points look like comet-shaped smears, and field curvature means the sharp image lies on a curved surface instead of a flat one. Astigmatism and distortion are the other off-axis aberrations.
Spherical aberration (a) occurs even for a point on the axis, because marginal rays focus nearer the lens than paraxial ones. Chromatic aberration (b) arises from dispersion and is present on axis too. Any option containing (a) or (b) is therefore wrong.
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