I posted a photo and tagged a friend. Her friend (whom I don't know) commented on it. I don't like that non-friends can somehow comment on my photos just because they are friends with my friend. Is there a way to block (at least) the comment part? Do I have to un-tag my friend to disallow comments?
If you post to "Friends" or more (including "Friends of Friends" or "Public"), then the friends of the people that are tagged will also be able to see, like, and comment on your post.
You can get around this by posting to a friend list instead. If you create a friend list with all of your friends on it and post to that list (by entering the list name using the Custom setting), then you can post to all of your friends without automatically including the friends of the tagged people. However when you add new friends you would need to remember to manually add them to that friend list.
You can control who you share with when posting each item separately. This is done through a dropdown list, as in this image:
Alternatively, if you want to change the default setting of who you share with, you can do so from your Privacy Settings screen. It is accessed from the top right corner, near your profile name.
Selecting Friends in any of the above will exclude your friends' friends, meaning they will not be able to access the photos, no matter who is tagged.