The most commonly used optical lenses are spherical lenses, named for their spherical surfaces. In such lenses, the optical axis is the straight line passing through the two centres of the calottes that define it.
The optical axis is the line passing through the centre of the radii of curvature of the surfaces and, in plano-convex or plano-concave lenses, is perpendicular to the surface and passing through the centre of the radius of curvature of the surface.
The centre of the lens is the point on the lens where light rays are not deflected. The most significant element of a lens is the focus, which is defined as the point where light beams that are travelling parallel to the optical axis will converge.
Its distance from the centre of the lens is called the focal length and is usually denoted with the letter f.
To construct the image of an object produced by a lens, one usually follows the path of two particular light rays: the first starts from the top of the object, passes through the lens parallel to the optical axis and is thus diverted towards the focal point, while the second ray starts at the top of the object and passes directly through the centre of the lens and continues onwards without any deviation. The intersection of the two rays defines the image point.