What is referred to as 'optical infinity' in optics?

In optics, 'optical infinity' refers to a distance at which light rays coming from an object are considered to be parallel, making it ideal for focusing in optical systems such as lenses. For practical purposes in vision care, 'optical infinity' is conventionally defined as a distance of 20 feet or more. This is because at this distance, any slight diverging rays from an object essentially become parallel, allowing the eye to accommodate less, facilitating a clearer focus.

The other choices, while they reference various distances, do not align with this established benchmark. The definition of optical infinity is rooted in both historical practices in optics and the behaviors observed in vision testing, particularly for individuals who require corrective lenses to see clearly at greater distances. Understanding this principle helps opticians provide appropriate prescriptions based on how clients see at different distances.