If you divide an image into a grid of 9 equal segments then you will have divided an image into thirds. By keeping your main subject away from the centre third then you have a more interesting shot. Place your most important subject at one of the intersection points then your shot will have more impact again.
When most of us are starting out in photography and we see a scene through the viewfinder or on a monitor we take the scene and if we have a definite subject it usually ends up in the centre of the frame. This is the way we see, it’s like our brain needs to have order. Unfortunately that approach often doesn’t work for the viewer of the photo. Even though our viewer is also like us and wants order, when they are looking at a picture it becomes quite boring to see order (this is all quite subconscious of course). On the other hand if the subject is placed to the left or the right rather than in the centre, or on one of the intersecting lines, somehow that creates disorder, which brings impact. This is called using the ‘Rule of Thirds’.
Using the rule of thirds is something to get into the habit of. When you are looking through your viewfinder or at your monitor when you are framing the shot then you automatically apply the rule of thirds to anything in the scene that is appropriate. Take a look at some of your own photos and see where you have or haven’t applied the rule of thirds and how can you improve some of those photos in the future? It may just be a matter of cropping so that a subject or a part of the image has been changed from say the central part to the left or the right.
To say that the rule of thirds improves the shot is correct but to say that one must apply the rule of thirds to every photograph is not correct. In fact some photographs lend themselves to a composition that does not contain the rule of thirds.
Your camera should have two useful tools to help you master the rule of thirds. Many cameras have the rule of thirds grid included in the display menu. Keep it switched on for a season. It will help you to remember the guidelines and understand the concepts. It will also help you to get your subjects placed on the intersections until you begin to think that way automatically.
Your next tool is focus lock. if you want to put your subject off centre just centre your subject initially and apply slight pressure to the shutter to lock the focus. Holding this pressure, re position your subjects at an intersecting point on the rule of thirds grid and press down all the way to take the photo.