The part confusing about opengl during studying the projection was that the focal length, top, left, bottom, right, wasn't declared, and it magically did it with FOV and image aspect ration. I found the wonderful maths in this amazing site, and found out how it is calculated based on FOV and focal length.
Two points I noted for my confusion was:
- near clipping plane is the plane which limits the projected coordinates, and
- focal length is the distance from the point of projection to the optical center( (0,0,0) in our case )
Now, the question is, is the focal length always equal to the distance of the near clipping plane from the optical center, i.e. in other words, is the image always projected on the near clipping plane (or at least in openGL)? Yet in other words, is the near clipping plane and the plane of projection the same thing just with different terms.