Final answer:
The question is related to the physics of the human eye's accommodation, where the eye adjusts the lens' power and focal length for clear vision. In terms of human vision, the minimum clear visual distance is typically around 25 cm for an individual with normal eyesight. The analogy alludes to the flexibility required for a mechanical optic system to permit free movement, similar to an eye lens adjusting to close objects.
Step-by-step explanation:
The query appears to revolve around the concept of accommodation in the human eye, where the power and focal length of the lens are adjusted by ciliary muscles to produce clear images on the retina for objects at various distances. When we talk about the lens acting as a lock and ensuring the dial (optic system) to be freely moving, it seems to be a metaphor relating to the flexibility of the eye lens adjusting its focus.
In human eyes, the nearest clear vision (near point) is typically considered to be about 25 cm for a person with normal vision, which is the distance the lens can comfortably accommodate to form a clear image on the retina.
For a lens, or a sight mechanism, to allow a dial to move freely would imply that the controlling elements (akin to the muscles in the eye) must release or adjust their hold to ensure full range of motion, analogous to the eye being fully accommodated when inspecting objects at close range.