This might be the most significant motif, or a running theme, in the entire novel. Given how huge this theme is, we're going to tackle four separate instances where sight comes into play in order to see just how varied this theme is in the novel.
First, we'll start off with the most notable example of the theme of sight: Clym's blindness. Sight is crucial to understanding Clym's character and his loss of vision is no accident on the part of the author.
Amid these jarring events Yeobright felt one thing to be indispensable – that he should speedily make some show of progress in his scholastic plan. With this view he read far into the small hours during many nights. [...] Fretting with impatience at this interruption to a task he was so anxious to hasten, Clym was transformed into an invalid. (4.2.36)
Clym got so focused on one thing that he lost sight of everything else around him. And this metaphorical shortsightedness translates itself into a literal blindness.
His daily life was of a curious microscopic sort, his whole world being limited to a circuit a few feet from his person. (4.2.57)
Clym certainly isn't the only character to have vision problems; in fact, many other characters suffer from similar sorts of myopia, or limited vision. These appear in different ways, though. Take Eustacia, for example – unlike Clym, who sees only what's immediately in front of him, or doesn't see at all, Eustacia sees things that aren't even there: "Involved in these imaginings she knew nothing of time. When she became conscious of externals it was dusk" (2.1.30). Eustacia's rich inner fantasy life tends to spill over into her actual life and literally disrupts her vision. As with Clym, the inability to see clearly causes a lot of anguish, or pain and suffering.
Yet another example of not seeing well comes with Mrs. Yeobright. In her case, she sees just fine, but she completely misinterprets what she sees, which causes a lot of problems too:
Her eyes were on the ground; within her two sights were graven – that of Clym's hook and brambles at the door, and that of a woman's face at the window. (4.6.64)
Mrs. Yeobright jumps to a hasty conclusion based on what she thinks she's seeing. And this tendency to misinterpret things or to draw hasty conclusions is a pronounced character trait in Mrs. Yeobright. We see this especially with her reaction to Clym's relationship with Eustacia – she instantly assumes the worst about Eustacia based on limited information and her own emotions, namely fear and jealousy.
So vision is a very unreliable thing in this novel. Characters constantly see the wrong thing or fail to fully understand what they're seeing, which is perhaps the most important thematic point here. Sight is only as good as someone's interpretation of it, and the characters here have a very hard time correctly understanding the things they see.