What distance is generally considered to be optical infinity?

The concept of "optical infinity" refers to a distance at which light rays entering an optical system are considered to be parallel. In optics, this distance is commonly recognized to be around 20 feet (approximately 6 meters). This is primarily due to the fact that at this distance, the differences in distance become negligible for the purpose of lens focus, and it allows for the simplification of calculations in optometry and vision science.

A distance of 20 feet is often used in various fields, including ophthalmology and optometry, to define the viewing distance for evaluating visual acuity, especially in the context of prescription glasses or contact lenses. Visual tests, for example, are usually conducted at this distance to standardize measurements of visual clarity.

The other distances provided, while they have their own contexts in specific fields or practical applications, do not align with the standard definition of optical infinity used in vision testing and lens design.