Focal length (f)

The focal length $f$ specifies the signed distance from an ideal thin lens to its optical focus for incident light rays parallel to the optical axis. A converging thin lens has $f>0$, with a rear optical focus beyond the thin lens; a diverging thin lens has $f<0$, with a virtual optical focus on the incoming side. The focal length is a property of the optical lens in its specified surroundings, not the image distance for every object. For example, a converging thin lens with $f=1\mathrm{m}$ images an object $3\mathrm{m}$ away at $1.5\mathrm{m}$, as computed below. ^[ref].

Focal length is a key concept in optics that refers to the distance between the lens or mirror and the point where parallel rays of light converge to a single point, known as the focal point. It is typically measured in millimeters (mm) and is a crucial parameter for both lenses and optical instruments, such as cameras and microscopes.