Hi, I studied concept of numerical aperture and f number I learned numerical aperture(N.A) is measure of how much light can be entered into the optical system, its mathematical expression is N.A = [tex]nsin\theta_m [/tex] where n is index of refraction of immersing medium surrounding lens and [tex]\theta_m[/tex] is maximum angle between optical axis and rim of lens. I got a question. What is the point from which two line(one is from the point to rim of lens, another is from the point along optical axis to lens) make maximum angle? I think maximum angle is varying as the point moves along axis, So it is not clear for me. And why can more light enter the optical system if n increases, causes N.A increase. Please solve my question.