Why can't I comment and thank Max C to tell him that his answer fixed my problem. I took the battery and card out, tried to focus again and it worked! Thank you, Max C!
Carol, you can confirm simply by voting the answer up, which you can do when you have a very little bit of positive interaction on the site. This is pretty easy to do, simply by asking a question of your own or providing a helpful answer to an unanswered question — existing users will vote it up. See more on this in the site help.
You don't need to comment. In fact, unless you have something substantial to add, it's better to not to. The goal of this site is to focus on the questions and answers directly, without extra chit-chat.