This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Elementary Optics, Subject name: Ophthalmology
[{'content': 'The optical procedure being done is used for: \nSelect and explain. \nA. Objective refraction, B. Subjective adjustment of cylinder, C. Binocular balancing of vision, D. Subjective refinement of cylinder', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. (d) Subjective refinement of cylinderRef: Khurana optics 2/e, p. 153The instrument seen in the image is the Jackson cross cylinder (over the trial frame) - which is used for subjective refinement of cylinder.', 'role': 'assistant'}]