Andrew Young's poem Field-Glasses is about what happens to the poet when he looks at a scene through a pair of field-glasses, which is another name for binoculars. This is an instrument which you look through in order to be able to see things that are far away.
You can immediately see how poets like to play with words on different levels. On one level, the poet is looking through an instrument called field-glasses. On another, he is literally looking at a field in the countryside through a pair of glasses. This kind of wordplay makes the poem interesting and also gives it a sense of depth.
A poem is all about trying to see things in a new way. This is also exactly the effect of looking through field-glasses. They magnify the view and so things that are far away seem much nearer. They make those things seem much bigger, and they also frame the view.
Your eyes are pressed up against the glasses, so you can see the view through the glasses but nothing outside of that view. If you move the glasses, then you get the sensation that the whole scene moves too.
So in writing about the view through the field-glasses, the poet is also writing about how your viewpoint changes in the process of writing a poem. Writing enables you to see things more closely. It focuses your attention on a scene in just the same way that a pair of field-glasses focuses your attention on the view that you are looking at.
What does Young see through his field-glasses?