The rule of thirds is a very important element of good composition not just in photography but in all visual arts. The idea behind this rule is interesting and almost esoteric but is absolutely necessary to master to take good photos. This is probably the most well known ‘rules’ of photographic composition. I’m also going to break from other guides on this rule and tell you that you absolutely must know when to break this rule in order to make compelling shots. It is a variation of the "golden ratio."
Our first record of the rule of thirds comes from the engraver John Thomas Smith. In his book Remarks on Rural Scenery, published in 1783. In this book he quotes Sir Joshua Reynolds and expands upon his idea that images should be split into three parts.
Analogous to this "Rule of thirds", (if I may be allowed so to call it) I have presumed to think that, in connecting or in breaking the various lines of a picture, it would likewise be a good rule to do it, in general, by a similar scheme of proportion; for example, in a design of landscape, to determine the sky at about two-thirds ; or else at about one-third, so that the material objects might occupy the other two : Again, two thirds of one element, (as of water) to one third of another element (as of land); and then both together to make but one third of the picture, of which the two other thirds should go for the sky and aerial perspectives. This rule would likewise apply in breaking a length of wall, or any other too great continuation of line that it may be found necessary to break by crossing or hiding it with some other object : In short, in applying this invention, generally speaking, or to any other case, whether of light, shade, form, or color, I have found the ratio of about two thirds to one third, or of one to two, a much better and more harmonizing proportion, than the precise formal half, the too-far-extending four-fifths—and, in short, than any other proportion whatever. I should think myself honored by the opinion of any gentleman on this point; but until I shall by better informed, shall conclude this general proportion of two and one to be the most pictoresque medium in all cases of breaking or otherwise qualifying straight lines and masses and groupes [sic], as Hogarth's line is agreed to be the most beautiful, (or, in other words, the most pictoresque) medium of curves.
The rule of thirds simply states that an image should be divided into a grid consisting of nine equal sized boxes. Essentially a Tic-Tac-Toe board, this method of framing is extremely simple to understand but can be tricky to master. When you align your subject on these lines, you create a better image. What is going on is that people find such compositions more "balanced" despite the counter-intuitive idea that moving something towards the edge of an image would have the opposite effect. The human eye has a certain way of navigating over an image and it doesn't just go to the center of the image. Instead the eye starts at the edges and works around the outside of the image before moving in for deeper inspection. By placing interesting nuggets of interest along those paths you can draw the viewer into your work as they survey it.
See, humans have this thing where we like things that come in threes. Every good bar joke starts with- "A priest, a rabbi, and a chicken walk into a bar." Duets are good but three people make a band. This rule is everywhere humans are. In fact once you start looking for this rule of thirds you may just begin to see it everywhere and in everything. Architecture is full of this rule. In the case here it is a stack of three bells. How often have you seen a building with three pillars across the front, three windows, or a combination of a pillar, door, pillar?
This grid forms the basis for almost all aesthetic principles in art. You can find this rule being used all over the world from ancient Greek and Roman art, to Michelangelo's frescoes, to modern buildings and the movie screen. Ideally you want your subject to rest on these dashed lines like a tightrope walker viewed from below, spilling into the boxes themselves.
Now you need to note that while traditional artists often used a 1:1 aspect ratio and could have rows and columns of squares, in digital photography we use a 16:9 or 16:10 aspect ratio. So our boxes are made up of rectangles consisting of 1/3 left to right, and 1/3 top to bottom. The ratio stays the same, the shape does not.
Let's look at an example of how framing alone along these lines can help an image greatly. I took a picture of a dog at the dog park and did so with the intention of being able to crop freely. I didn't do any other editing.
In this first image the dog is perfectly centered. While technically following the rule of thirds in that the dog takes up boxes 2,5,and 8, she is perfectly centered. You have your framing from left to right, empty, dog, empty. This isn't good, there is some bad visual tension.
If we move this dog to the top left. at the intersection of three boxes we get a much better composition. For lack of a better term we give this dog more room to run. The image is balanced.
It's also important to try to get your living subjects eyes into one of the intersections. The reason for this is because our eyes are drawn to the intersection areas and move from there. So by putting the eyes there we can create an instant connection. The eyes truly are the window to the soul, so by showing the eyes first we can manipulate the emotional response of the viewer. For instance if the fierce eyes on an angry dog are seen first, the viewer could feel their flight response kick in. On the other hand a happy dog bearing his fangs with a giant smile and glowing eyes will warm the heart. The problem is the fangs, if the viewers eyes are drawn to them then you may not be invoking the right initial impression and feelings.
Consider this bird. The eyes fall very close to the top right intersect. In fact we could crop a bit to make it precise. The angle of the head and eyes create a mellow and delighted look that transfers to the viewer. This works well to draw the viewer into what would otherwise be a very center weighted image.
This picture is another great example. As you can see the right eye is dead center in the crosshairs. The left eye is looking right at another intersection. This makes us look at the African orphan in this photo right in the eye, and before we can turn away we have already made the investment of looking into his eye. Such a tool as this rule can be used to force the viewer to view an image that would otherwise be hard to look at.
What happens if your subject is too big to stick towards the corner, for instance a skyscraper or large tree? You can still follow the rule of thirds by using it vertically.
Notice how this Hoodoo rock formation takes up the entire center of the view? This is great composition because your eyes start at the outside but see nothing but sky until they rest on rock. The left and right sides of the image create negative space. But still the rule of thirds is followed.
By now though you are probably wondering when you should break the rule of thirds. There are times where using the rule of thirds will hinder the impact of your photo. Lets let the master Ansel Adams show us how it's done.
Hey! Wait! That horizon line takes up less than 1/5 of the image and that cone is dead center. What gives? Well remember when I said that the stuff that falls on the grid lines is going to draw interest? Adams doesn't want to draw interest to the horizon, and as interesting as the water could be it doesn't allow him to show the shape of the cone if it's too far away or off to the side. In the name of saving the subject Adams composed the shot so that the cone was dead center and everything else was secondary.
In this example of my work, the ground does the same thing that the sky does for Ansel Adams. I want the clouds and strata to be the subject. If I follow the rule of thirds I will lose a lot of that sky. But if I cut off the ground in framing my shot will feel strange with no context. I must downplay the ground! I turn the ground into a strip along the bottom and the shot is successful. What is going on here is that the rule of thirds would be harmful to your composition. This does not mean that we throw composition to the wind. Rather a more complex rule is necessary. In this case the rule we use is the golden ratio applied horizontally. Look carefully! The ground is cut into at least four layers as you look from bottom to top. The blue sky is another two layers. We aren't even a third of the way up and already we have tons of lines dividing the picture. When we get to the middle of the image things begin to blur and become much less defined.
Sometimes the whole rule of thirds must be disregarded because of the limits of composition and framing. You must learn to do this too. The saying "Rules were meant to be broken" totally applies here. Rules help you but they also restrict you. Don't get predictable by following them religiously.
Sometimes you should aim for switching to the more complicated "golden ratio." The golden ratio is a bit more complicated to wield but is simply a more complex system than the rule of thirds. As you can see the spiral shrinks greatly at the bottom right corner where the intersects using the rule of thirds would fall. This is for another day though.
Your DSLR and smartphone will usually both have the grid necessary to follow precisely the rule of thirds. Use them! They are there to help you frame using this rule.