Wait, slow down. What is "aperture" again?
Simply put, it's the opening in your lens that allows light to enter. Depending on how big or small the aperture (or opening) is, the more or less light that can enter.
Well, that seems pretty straightforward. Is this a trick?
Nope. It is actually pretty straightforward. Remember our window analogy? Just like a large wide open window allows light to flood into a room and a tiny window lets in only a small amount of light, so it is with large or small apertures. The tricky part comes when we talk about the size of the opening in terms of f-stops. At first glace, f-stop numbers make no sense whatsoever since there's no easily identifiable pattern. Add to that the fact that small numbers mean bigger openings and big numbers mean smaller openings and it's easy to see why people get so confused. BUT! If you think about what an f-stop actually is, it's much easier to remember which numbers correspond to larger/smaller openings. The f-stop is not just a random number, it's a ratio. (Technically, it's the ratio between the diameter of the lens aperture and the focal length of the lens, but you honestly do not need to remember that). And, like all ratios, it can be written like a fraction.
Uh, okay. I see why you titled this post "The Art & Science of Aperture" because that definitely got a little math/science nerdy for a minute. So how does the "art" play into all this science stuff?
Now that you know the science behind aperture, you can use it artistically in many different ways. For example, low light situations. Sometimes you want to take pictures when it's not bright and sunny, and you don't always want to use a flash to do so. One of your options would be to use a larger aperture. Remember, the larger your aperture the more light that will get through. If there's not much light available to begin with, you need to open your aperture nice & wide to let as much in as possible.
Likewise, sometimes you do want to make a nice photo when it's bright and sunny but you don't want to blow out all the brightest spots. Fast shutter speeds and low ISOs are great tools, of course, but another option would be to increase your f-stop and create a more narrow opening to minimize the amount of light entering the camera.
f/1.4 - It was late afternoon, and the window was the only source of light in the brewery room. I wanted the glass of beer to be properly exposed and the bottles in the background to be dark but visible. To capture the minimal light hitting the front of the glass (not the back light from the window), I needed a larger aperture.
f/11 - This was by far one of the brightest/sunniest days of our trip and this was shot around midday (hence the minimal shadows). The narrow aperture helped keep the lighter patches of grass from blowing out under the direct sun.
Any other artistic uses for aperture?
Of course! My favorite use of aperture is to control depth of field and create beautiful background blur. Depth of field simply means the area within a frame that is in focus. Larger apertures (i.e. small f-stop numbers) create shallow depth of field so only a narrow part of the frame will be in focus while the foreground and background will be blurry. Larger apertures are great for portraits, for example, when you want to focus on the eyes but soften the other features just a bit.
On the flip side, smaller apertures (i.e. high f-stop numbers) create larger/deeper depth of field so that the majority of the frame, from foreground to background, will be in focus. One great example of a time to use smaller apertures would be for landscape photography, when you want the full scene to be clear and visible. Here are a few examples from our Argentina trip illustrating different uses of depth of field.
f/1.8 - The "nt" of Tante engraved on the cup is nice and clear but since the cup is curved, focus already softens at the "F" of Frida.
f/4.0 - Because the camera is just slightly angled from the house, the narrower aperture was enough to bring most of Casa Rosada into focus.
f/7.1 - The pathway in the foreground is mostly in focus, creating leading lines to the house, but there's still some depth and separation from the trees in the background.
f/9.0 - My focal point was placed pretty far out in the distance but this aperture setting allowed for the shoreline to be (mostly) clear.
I hope this gives you a little bit of a better understanding of how aperture works and the various ways you can use it creatively. If you're just starting out, I'd recommend putting your camera in Aperture Priority mode (Av or A on the dial) and focus on just shooting at different apertures for a while. Once you get the hang of it, you can worry about the other settings and get even more creative with it in manual. Experimenting and playing is really the best way to understand how it all works. And with enough practice, the numbers thing won't be confusing at all!
(All the photos in this post were shot with the Canon 35mm f/1.4.)