Question

what is principle focus of a lens​

Answer 1

HOLA MATES

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by aberrations of the imaging optics. In the absence of significant aberrations, the smallest possible blur circle is the Airy disc, which is caused by diffraction from the optical system's aperture. Aberrations tend to get worse as the aperture diameter increases, while the Airy circle is smallest for large apertures.

Answer 2

In a lens there is two foci and it is when we complete the two circle from both arc like side of lens we get 2 centre and the mid point between the two centre from pole is the 2 focii and when we put an object at infinity on one side its image formed on other side focus and that focus for that time called principle focus