Light has limits, even for elves. Light is corralled by the fabric of the universe to never exceed 186,000 miles per second and by the structure of humanoid eyes to only resolve images from so far away. Light’s limits are why Legolas would have a hard time picking out Rohirrim riders from leagues away.
As Henry Reich of the immensely popular science YouTube Channel Minute Physics explains in the video below, even assuming Lord of the Rings’ Legolas has what are basically perfect human eyes, light’s behavior as a wave puts a firm limit on the distance at which he could still clearly see:
Legolas could increase his visual acuity if he had larger eyes or could detect shorter wavelengths of light, as Reich explains, which isn’t that far-fetched considering the eyes of other animals. Some insects such as bees can see in ultra-violet light (shorter wavelengths than visible light), and eagles could make out an ant at 100 feet (30 meters) up with how good their vision is.
Unless his eyes were cheated by some spell, a slightly enhanced Legolas could very well pick out blonde equestrians at 15 miles (24 kilometers) away. But because of the way waves of light move through small holes, Orlando Bloom might as well have been looking at a blur.
Of course, a simpler explanation is magic.
IMAGE: Warner Bros.