Which statement about longitudinal spherical aberration is NOT true?

Which statement about longitudinal spherical aberration is NOT true?

• A. It is a type of spherical aberration
• B. It occurs for off axis points only
• C. It occurs when marginal rays focus at a different point compared to paraxial rays
• D. It contributes to nocturnal myopia

Answer: (B)

Longitudinal spherical aberration describes an axial difference in where rays come to focus for a point on the optical axis: the paraxial ray and a marginal ray do not meet at the same point along the optic axis. This creates an axial blur because the image of a single point spans different focal depths depending on ray height. It’s a consequence of using a spherical surface and can occur even for an object point on the axis, especially as the pupil enlarges and peripheral rays contribute more. In other words, it’s not restricted to off-axis points; the axis-aligned point’s focus is what defines the longitudinal component. The other statements are true: it is a type of spherical aberration, it involves marginal rays focusing at a different axial point than paraxial rays, and larger pupils in low light can worsen it, contributing to nocturnal myopia.

Longitudinal spherical aberration describes an axial difference in where rays come to focus for a point on the optical axis: the paraxial ray and a marginal ray do not meet at the same point along the optic axis. This creates an axial blur because the image of a single point spans different focal depths depending on ray height. It’s a consequence of using a spherical surface and can occur even for an object point on the axis, especially as the pupil enlarges and peripheral rays contribute more. In other words, it’s not restricted to off-axis points; the axis-aligned point’s focus is what defines the longitudinal component. The other statements are true: it is a type of spherical aberration, it involves marginal rays focusing at a different axial point than paraxial rays, and larger pupils in low light can worsen it, contributing to nocturnal myopia.