Legolas' sight far exceeds Aragorn's

The ridge upon which the companions stood went down steeply before their feet. Below it twenty fathoms or more, there was a wide and rugged shelf which ended suddenly in the brink of a sheer cliff: the East Wall of Rohan. So ended the Emyn Muil, and the green plains of the Rohirrim stretched away before them to the edge of sight.

‘Look!’ cried Legolas, pointing up into the pale sky above them. ‘There is the eagle again! He is very high. He seems to be flying now away, from this land back to the North. He is going with great speed. Look!’

‘No, not even my eyes can see him, my good Legolas,’ said Aragorn. ‘He must be far aloft indeed. I wonder what is his errand, if he is the same bird that I have seen before. But look! I can see something nearer at hand and more urgent; there is something moving over the plain!’

‘Many things,’ said Legolas. ‘It is a great company on foot; but I cannot say more, nor see what kind of folk they may be. They are many leagues away: twelve, I guess; but the flatness of the plain is hard to measure.’

The Two Towers, Book Three, Chapter 2: The Riders of Rohan.