In the world of Anna Karenina, the eyes of Leo Tolstoy see all and know all. In other words, this novel is told from the perspective of an omniscient, or all-knowing third-person narrator. The story slips into the perspectives of Anna, Vronsky, Karenin, Levin – even Levin's dog, Laska (for two chapters)!
If there's no limit to what this narrator can describe, we have to be interested by what the book doesn't tell us. For example, in the concluding chapters of Part 7, we get a lot of introspection from Anna, but very little from Vronsky. What does this choice do to our sympathy for these characters? Or, how about the fact that Part 8 focuses on Levin instead of, say, Karenin, Seryozha, or even Vronsky? This narrator has proved that he can give us insight into anyone in the novel. We have to wonder why he chooses one character over another.
This leads us to an important point about Anna Karenina's omniscient narrator. As readers, it might seem to us that, because the narrator can tell anybody's story, that he's an uninvolved or objective narrator. But while the narrator can be anywhere in the novel at any time, the fact that he sometimes chooses to give one person's perspective instead of another's is also partisanship. The narrator does have a message to get across, but Tolstoy's too subtle to come out and say, "Here it is! Pay attention here!" Instead, he lets the perspectives of the characters of most interest to him ethically or psychologically dominate the novel. So, Anna Karenina's narrator may seem objective, but if you really look closely, he's anything but.