The muscles of a normal eye experience the least strain when...

When a normal eye focuses on an object placed at infinity (far away), the ciliary muscles are completely relaxed. In this state, the focal length of the eye lens is at its maximum, and the eye is in an unaccommodated state.

Conversely, when focusing on nearby objects, the ciliary muscles contract to increase the curvature of the lens, which causes strain. The maximum strain occurs when the object is at the near point (about $25 \text{ cm}$).

Thus, the muscles experience the least strain when the object is far away from the eye.