For a simple lens, focal length is measured from the lens’s optical center to the point where light from a very distant subject comes to focus. In real camera lenses, the exact internal reference point is more complex, but the idea is the same.

The sharp image is formed on the image plane. In a DSLR, that is either:

• the sensor/film when taking the photo, or
• the focusing screen when viewing through the optical viewfinder.

The mirror directs light upward to the focusing screen, and the camera is adjusted so the focusing screen is the same optical distance from the lens as the sensor. That’s why an image that looks in focus in the viewfinder is also in focus on the sensor.

The “point” where rays meet is a simplification. A real image is formed across a plane, not just a single point for the whole scene. If the sensor or focusing screen is placed at that image plane, the image is sharp; if not, it looks blurry.

A lens with a shorter focal length gives a wider angle of view; a longer focal length gives a narrower angle of view and greater magnification, which we call telephoto.