This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Optics and refraction, Subject name: Ophthalmology
Pin hole can reduce refractive error upto - 
Select and explain. 
A. 1 D, B. 3 D, C. 5 D, D. 10 D
B: 3 D
Answer: B: 3 D 
Explanation: Looking through a pinhole reduces the patients refreactive error and improves vision by increasing depth of focus. However, the pinhole vision is limited by diffraction. The opitimal apeure size of a pinhole is 1.2mm, which may correct up to 3D of refreactive error. Reference : Essentials of ophthalmology by Kaiser p.20