Optical terminology: what does “aplanatic” signify? A lens (or lens combination) is termed aplanatic if it completely eliminates which optical defect?

Introduction / Context:
Surveying instruments rely on lens assemblies designed to reduce image defects that blur lines and graduations. The terms aplanatic, achromatic, and anastigmatic describe which aberrations are corrected by a given design. Knowing these definitions helps interpret instrument specifications and performance limits.

Given Data / Assumptions:

• Standard optical nomenclature is used.
• “Aplanatic” relates primarily to spherical aberration correction; in many contexts it also implies freedom from coma at a particular aperture and field point.

Concept / Approach:

Spherical aberration arises because rays at different heights from the axis focus at different points when surfaces are spherical. An aplanatic lens system is designed so that these rays meet at one focus, eliminating spherical aberration (and often coma for selected field points). Other aberrations—chromatic, astigmatism—require additional design features and are not inherently removed by aplanatism alone.

Step-by-Step Solution:

Verification / Alternative check:

Textbook glossaries define aplanatic as “free from spherical aberration” (and sometimes coma at a particular zone), distinguishing it from “achromatic” (color correction) and “anastigmatic” (astigmatism correction).

Why Other Options Are Wrong:

Chromatic aberration relates to wavelength dependence; coma is an off-axis blur; astigmatism produces line foci—none are implicitly eliminated by aplanatism.

Common Pitfalls:

Confusing aplanatic with achromatic; assuming a single term implies complete correction of all aberrations.

Final Answer:

Spherical aberration