I used to do this a lot in Cornerstone when I was working with SVN. Select a single file and then view the log. It would show me only changesets that affected the selected file, so I could quickly find out when a change was introduced and the surrounding context.
I can't find any way to do this in SourceTree. I've tried using blame on the file, then opening the changeset for the line I'm interested in, but once I do this I can't find any way to go back further into the history. When the actual change I'm interested in wasn't introduced in the most recent change to that line (e.g. subsequent unrelated changes, or code style changes), it makes it difficult to step back any further and find the change I'm interested in.
Am I just missing something in SourceTree, or Git itself that?