# [Q] Do you understand what is numerical aperture and f number?

1. Apr 7, 2009

### good_phy

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 = $$nsin\theta_m$$ where n is

index of refraction of immersing medium surrounding lens and $$\theta_m$$ 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.

2. Apr 7, 2009

### Andy Resnick

I'm not sure I understand your question, but the cone angle used in the definition of NA has the apex located at the focal plane of the lens, not arbitrarily located along the optical axis.

Does that help?