You can easily reset the camera to its original position. Doing this positions the camera precisely where you want it to be rather than gauging it visually -see Positioning an Element Using the Layer Properties View. Another way to set up your camera frame is to type the coordinates directly in the camera's properties. You can reposition your camera frame directly in the Camera view, using the advanced animation tools. You could use this feature if you are still working on your scene composition and have different camera framing to try out. If you add several cameras to your scene, you can use the Camera list to select the active one. You need to add a camera layer to your scene, so you can edit the camera frame. The Camera layer is static which means that if you need to animate it, you must add a peg -see Animating the Camera and 3D Camera Motion. If you're working in a gaming animation pipeline, you also have to possibility to set an orthographic camera -see Orthographic Camera. You can adjust the camera resolution and other parameters in the Scene Settings window -see Scene Settings.
The scene action occurs inside the camera frame, so it’s really important to set it up correctly.