The focal length is positive for a converging lens but negative for a diverging lens, giving a virtual focus, indicated by a cone of gray rays.

The lens index of refraction is given by . Optical-quality glass has in the vicinity of 2.65. The top slider enables you to vary between 1.0008, its value for air, and 3.42, the refractive index of diamond.

The width represents the distance between the faces of the lens along the optical axis. The value of is restrained by the slider so that the lens faces never intersect anywhere.

The parameters , , and are to be expressed in the same length units, often cm. The reciprocal is known as the optical power of the lens, expressed in diopters . A converging lens, as shown in the thumbnail, can serve as a simple magnifying glass.

In the thin-lens approximation, the lens width is small compared to the other lengths and the lensmaker's equation can be simplified to .