Question

how can we find out the focus of a lens ​

Answer 1

Answer:

The greater effect a lens has on light rays, the more powerful it is said to be. A powerful converging lens will focus parallel light rays closer to itself and will have a smaller focal length than a weak lens. The power of a lens is given by the equation P=1f P = 1 f

Literally, the focus is the focal point. Take the lens off the camera, hold it near a flat surface in the sunshine, and adjust it until you get a sharp image of the sun on the surface. That's the focus, and it's at the focal length from the optical centre of the lens (so typically in the range 30 to 300 mm).