A remote pilot is flying a small unmanned aircraft system (sUAS) over a large, featureless open field during daylight hours. The pilot reports difficulty maintaining a sharp focus on the sUAS when it is hovering approximately 500 feet away, even though flight controls indicate a stable hover. Which physiological factor affecting vision is most likely contributing to this issue?

A remote pilot is operating a small unmanned aircraft over a large lake with few nearby visual references. This scenario is most likely to cause which physiological effect on the pilot's vision?