The focal length of a lens is the distance from the center of the lens to the focal point, where parallel rays of light passing through the lens converge. This distance depends on the shape and material of the lens.
Lenses are categorized as converging (which bring light rays together) when their focal length is greater than zero (F > 0) and diverging (which spread out light rays) when their focal length is less than zero (F < 0).
In thin lenses with a small radius of curvature and thickness, the focal length can be calculated using the formula:
where
F – focal length,
n – refractive index of the lens material,
R1 and R2 – radii of curvature of the lens on both sides.
In more complex lenses, such as thick lenses or lenses with a large radius of curvature, more intricate formulas are used to calculate the focal length.
The focal length is an important characteristic of a lens because it determines the scale and resolving power of the optical system it is a part of. For example, short-focal-length lenses have a smaller focal length and larger scale, making them suitable for creating wide-angle images, while long-focal-length lenses have a greater focal length and higher resolving power, making them suitable for producing highly detailed images.