In manual photography, you learn to think about many things. From a technical standpoint, you want to think about achieving perfect exposure. As you may know, exposure is made from three important pillars: Shutter Speed, Aperture, and ISO. Now, the true art of photography is not just learning how to use these in concert, but using them to your advantage to translate the visualization of the image you want into reality. Today, I'll focus on Aperture.
It's all in the details.
Using the simplest terms, aperture just means opening. A way for light to travel through the lens. Now, the aperture on the camera isn't so simple, but it has only one mechanic: open and close. Some lenses open wider, some close tighter. This is primarily to let in more or less light, to affect the exposure of the overall image. But, this is not the true purpose of the aperture, only it's relation to exposure. The real purpose, and reason the aperture gives you creative control is much more interesting.
Color and Rust.
When you take photos of still life subjects, be they flowers, trees, buildings, animals, even models, the first thing you want to consider is what your focal point should be. Imagine a table with a lamp on the far edge, a small porcelain figurine of a cat a few inches in front of it, and in front of the cat, a stack of pink erasers almost touching the front edge. You decide that you only want the porcelain cat to be your focal point, and you don't want anything else to be in focus. That last part, not wanting anything else in focus, that means we want a "shallow depth of field". It's an intimidating, technical term, but its very simple to visualize.
Starting with a low aperture value, aka f-stop, say f1.8, you find that if you focus your lens manually, very few things are in focus at a time. As you bring your focus one way, you'll see the erasers are in focus, turn it all the way the other way, the erasers turn into pink blobs, but the lamp shows definition. When you turn it back, the porcelain cat becomes in focus. Now, what do you do when you want everything in focus? Well, this is when you change your f-stop, thus changing the depth of field, or the about of objects in focus at once. As you change the f-stop, the aperture is shrinking to a smaller hole. But as the size of the aperture shrinks, less light comes into the lens, and affects your exposure, but we won't get into that too much right now.
When the light comes into your camera through a narrower hole, it brings things that were once out of focus, into focus. So if you move your f-stop to f8, you will see that now the pink erasers are in focus, along with the porcelain cat. Move it again to f16, and now the lamp is in focus, without even having to move the focus ring one bit. I find it helpful to visualize in my head that my depth of field is a three dimensional slice, almost like a slice of bread. At f1.8 the slice is very thin, and only highlights the porcelain cat. At f8, the slice is bigger now, so it encompasses the porcelain cat, and the erasers. If you were to move your focus ring, you'd be able to move focus from the erasers and porcelain cat to the porcelain cat and lamp. Imagine just moving the slice from one point in space close to you, to a few inches further into space. The slice doesn't change size unless you change the aperture.
Museum of Non Flying Planes.
So, how does this relate to you and your photography? Well, like I mentioned before, if you're taking photos of objects that are relatively still, you will want to think about aperture first. When you first start out with photography, it's tempting to think only about exposure, but the way you find your voice is making decisions based on your vision, what your eye is drawn to, not just what gives you perfect exposure. So if you are taking photos of a row of tulips, you may decide to only focus on one, two, or even the entire row. There's no right way to take the photo, so it's your artistic license to choose.
Lost in a sea of concrete and sky.
Now that you've mastered understanding aperture and depth of field, let me give you another nugget of knowledge. The relative distance of the camera of the subject does effect depth of field. The closer you are to the object, the smaller the aforementioned slices become. Imagine you're taking a photo of a face, and you're right up in the subject's grill. At a very low f-stop, say f1.8, your slice becomes razor thin, to the point that you might only be able to focus on the nose, the eyes, or the ears, but not all at once. If the subject takes a few steps back, you'll notice that the subject's face is now completely in focus. This is due to the way light travels through the lens, which is a fairly complicated physics function, but I won't go into that.
The best way to really grasp the above information is to go out and experiment! Set your camera to the lowest f-stop value, be it f1.4, f2.0, whatever is smallest, and switch to manual focus. Take photos of objects on a table, take photos of flowers in your garden, take photos of buildings and experiment with high apertures like f10, f16, even f20! Find what kinds of depth of field you like best, and sound off in the comments!