When you have the Camera app open and the desired image displayed on the screen, you can change the focus area of the image before you take the picture. Simply tap on the portion of the image on which you want the camera to focus. The camera automatically adjusts the focus and exposure for that area. You can use this to your advantage if you don't like the lighting (darkness or brightness) of the image. Simply tap on different areas until it looks the best. Then tap on the camera icon to take the picture.
You can also use this feature when you are taking close-up photos (i.e. photos of people or objects less than a foot from the camera). Align the photo and tap on the image until the foreground image looks best against the background. Then take the photo.