The focal point of a lens is defined by considering a narrow beam of light incident upon the lens, parallel to the optic (symmetry) axis of the lens and centered on that axis. The focal point is that point to which the rays converge or from which they diverge after passing through the lens. The convergent case defines a converging (positive) lens. The second case defines a diverging (negative) lens. It’s easy to tell which kind of lens you have, for converging lenses are thicker at their center than at the edges, and diverging lenses are thinner at the center than at the edges.
The point or distance from the instrument at which the object is focused onto the detector within the instrument. The focal point is the place or distance at which the optical or infrared resolution is greatest.