For a given optical system, there always a term called resolution power. It is called limitaion caused by diffraction.

 

 

I have known the equation

sin(theta)=1.22lambda/ampture.

But not really understanding the meanings.

 

Can anyone give a comprehensive explanlation about the resolution power or diffraction limitaion?

Yes, the Rayleigh criterion. My explanation will probably not be as good as one written and edited, so I'd suggest reading:

 

http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/Raylei.html

 

And then coming back and asking any questions you have

Very roughly speaking, a point source will get smeared out by diffraction when you image it. When two sources next to each other can't be resolved because of this, that's your limit.

what is the angle theta refer to?

Can anyone give me a graphic representation for the angle theta?

Thanks in advance.

theta is the minimum angle between two point sources that can be resolved...

 

I can't find an image for it, but if you were to draw a ray from each source to the centre of the lens then it'd be angle between the two lines.