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
The optical procedure being done is used for: 
Select and explain. 
A. Objective refraction, B. Subjective adjustment of cylinder, C. Binocular balancing of vision, D. Subjective refinement of cylinder
D: Subjective refinement of cylinder
Answer: D: Subjective refinement of cylinder 
Explanation: 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.