"Learn the rules like a pro, so you can break them like an artist"
-- Pablo Picasso
I was going to start this post by saying something about photography not having many rules, but stopped myself. It sure does - maybe that's why I like it. There are basically rules about shutter speed, aperture, exposure, etc. because we are attempting to capture something external to the lens through a sensor. If it's not captured with a certain sharpness and shape, there's nothing good to see. But within those rules, there is still so much room for creativity in subject matter, lighting, colors, and composition.
So, let's talk about the most famous rule for composition - the Rule of Thirds. The Rule of Thirds maintains that an image is more pleasing to the eye when its main focus sits on one of the points where lines dividing the horizontal and vertical of the image intersect, or on one of the lines - vertical or horizontal. Exactly where this is varies based on the dimensions of your aspect ratio. The aspect ratio is the proportion of the image you are capturing or creating. Most of what we shoot with our cameras or cell phones are at a 2:3 (or 4:6) ratio or a 16:9 ratio. (Confusion warning! The first is height x width and the second is width x height. ARGH!!!).
First, let me help you "cheat" a little. Most phones and cameras give you an option to turn on these gridlines when you are shooting a pic. I find them to be a nice way to visualize.
Android phone: Go to Settings (Mine is in the camera app > More > Settings). Choose something like "Grid Type" and select 3x3. On my iPad, I go into the device settings, find Camera and turn "Grid" on. And each camera will be different - Google or check your manual.
Next let's talk about which of the points to choose...
Landscapes:
Decide if you want your sky to be the main focus or not and put your horizon or main element of the landscape on one of the two horizontal lines. If you want to show a lot of sky, put the edge of the sky on the lower grid line. If not, put it on the top line.
If there is a specific point of focus, align it with one of those intersecting points - or at least close to one.
This doesn't have to be exact - I've picked examples that are pretty close, but there are plenty of photos in my portfolio that are close vs exact. Go with what pleases your eye.
You can see a couple ways of doing this in the images below. In the one on the left, I intentionally lined up my bottom third with the floor of the woods and my top third with some of the trees. In the other, I really wanted to show the dramatic clouds so the edge of the sky is aligned with the lower line,
Pics with a clear main subject:
Try to aim to get the main subject at one of the focus points.
Generally, have your subject looking into, or moving into, the frame vs looking or moving out of it.
I typically align a face with one of the top two intersecting points so that I show more of the body in the lower part of the frame.
In the egret picture, I have him flying into the frame. And I typically would have put the moon higher in the frame, but in this case I wanted to include the interesting clouds above it.
In each of the images below, I lined up the eyes on the top line. The pics would be different if I'd lined them up on the intersection point, but in each case, I actually preferred this composition.
In this image, I centered the car, but aligned the headlights in the upper grid line. Interestingly, they are also on the intersection points, but that was by accident vs intentional.
I will let you in on a little secret (shhhh...). I often don't capture things this well composed in camera. Those pesky wild animals don't always stay where I want them to. Or I may have my focus point in the middle of the frame and I don't have time to change. Or I couldn't get my shot close enough and know that I will need to crop later. In all those cases, I spend time considering the Rule of Thirds when I edit - either in my phone or on my computer. Every editing program will overlay this grid when "crop" is selected. It's a great tool for composition.
Now Break the Rule!
You've heard the saying that every rule has an exception. I do break the rule, intentionally, on a fairly regular basis. Sometimes I want to convey movement out of the frame, like in this image of the Caspian Tern. The water splashes roughly intersect with the bottom third of the frame. but he's definitely flying out of frame vs into it. In this case, the most interesting part of the picture story is where he'd BEEN vs where he is going.
In some cases I choose a central line instead of the Rule of Thirds to reinforce a sense of symmetry. The reflection image is an example of that. Breaking the rule helped me to create the mood I wanted and draws the eye to the magic of the reflection.
So here's my advice... understand the Rule of Thirds and how it works. But be willing to consciously break the rule when it suits your eye. Have fun!