Aperture in photography: an essential concept

Recently, I decided to start writing again about the basics of photography, which I think is a good resolution for my travel and photography blog. Some time ago, I published a basic article that everyone who wants to improve, progress and learn photography should read first: exposure in photography. I invite you to read (or reread) it before starting this more technical and precise article. Here, I’ll discuss one of the three main parameters of exposure: aperture in photography. For me, aperture is the most delicate exposure setting to understand. The other two are, in my opinion, easier to grasp. Even if the concept may seem a bit abstract at first, once it is clearly defined and explained, and with a little practice, the idea becomes fairly simple. I hope this article will help you understand how to take better photos, which settings to use, and how to get out of automatic mode on your digital camera. If you bought a DSLR or mirrorless camera, you might as well learn a little technical photography, right? Soon, shutter speed, long exposure, light and bokeh will no longer hold any secrets for you!

As explained in the article mentioned above, aperture is one of the three settings that lets you change the final exposure of your image. The trio “shutter speed / ISO sensitivity / aperture” is essential if you want to control your photos. The goal, together with the other two settings, is to get properly exposed images by deliberately controlling the amount of light that reaches your sensor. The key is to avoid, unless you are doing it intentionally, an overexposed photo that is too bright or an underexposed photo that is too dark. However, keep in mind that aperture is also one of the most important settings for adding creative effects to your images, such as background or foreground blur. We’ll come back to this at the end of the article.

Let’s get straight to the point. Strictly speaking, instead of talking about aperture in photography, we should talk about the opening of the diaphragm. The diaphragm is a part of your lens made up of circular blades. Aperture simply refers to the diameter of that opening. That is the main idea to remember.

Terms and notation

Aperture is written as an “f-number”. For example: f/1.8, f/11 or f/13. The number after “f/” corresponds to the aperture value you have chosen, in other words the size of the diaphragm opening. This is called an “f-stop”. So far, simple enough, right? Things become slightly more confusing because aperture notation works in reverse compared with the actual size of the diaphragm opening. Okay, I may have lost you already. What you need to remember is this: the smaller the number after f/ (for example f/1.4), the larger the aperture. In other words, the hole is bigger. Conversely, the larger the number after f/ (for example f/13), the smaller the aperture. No need to go into the technical reason behind this quirk here. Just remember this:

Reading aperture on a lens

To make this more concrete, every lens you buy shows its maximum aperture, along with its focal length. This aperture can be constant, for example 70-200 mm f/4, or variable, for example 70-300 mm f/4-5.6. When there is only one number after “f/”, it means the maximum aperture stays the same whatever focal length you use. If there are two numbers after “f/”, it simply means that the maximum aperture changes depending on the focal length. To put it simply: the more you zoom in with this type of lens, the less light will reach your camera sensor. So your image will be darker, which is annoying because you will then have to compensate with one of the other two exposure settings.

The notion of light

Aperture therefore influences the amount of light reaching your sensor. Keep in mind that the smaller your aperture is (f/11 or f/14, remember, it works in reverse), the less light reaches your sensor, and vice versa. You’ll often hear people talk about “stopping down” or “closing the diaphragm”, for example going from f/8 to f/11. In this case, you have closed the aperture by one stop, and the amount of light has been halved. On the other hand, going from f/5.6 to f/4 opens the diaphragm by one stop and doubles the amount of light reaching your camera sensor. I’ll explain in the next section how to change your aperture in a little more detail. For now, remember that the three exposure settings are linked. If you change the aperture, you change the final exposure of your image. To keep the same exposure, you will have to adjust one of the other two settings: ISO or shutter speed. This is a very important concept to understand: the three settings are all connected. By the end of the article, you will also understand why a lens with a maximum aperture of f/2.8 can cost much more than the same type of lens with a maximum aperture of f/4.

We have just covered the theoretical basics of aperture and the related concepts. Now let’s look at the practical side: how do you actually change the aperture of the diaphragm? No need to be a genius here. You have two main ways to set and change your aperture. Manual mode: this is the most difficult mode to understand, and I don’t recommend it for beginners until you have fully grasped the connection between shutter speed, ISO and aperture. Depending on the camera, you usually hold down the “Av/A” button or use a dial to adjust the aperture. “Av” mode / aperture priority: this mode is suitable for beginners and, honestly, for everyone else too (I still use it a lot). The principle is simple: you set the aperture by turning the dial, then set the ISO, or leave it on Auto ISO, and the camera automatically calculates the most suitable shutter speed for the shot.

What to remember when opening or closing your diaphragm: I already said a few words about this in the article on exposure, but I’ll repeat it here. The system works in “stops”. Most current cameras also offer intermediate settings, often in thirds of a stop. The important thing to understand is that each full stop either doubles or halves the amount of light. Let’s take the example of going from f/16 to f/11. You will often need to turn the dial three clicks to go from f/16 to f/11, meaning three thirds of a stop: f/16 to f/14 = one third, f/14 to f/13 = one third, then f/13 to f/11 = one third. In this case, you have doubled the amount of light.

As I mentioned earlier, the aperture of the diaphragm does not only affect the exposure of your photo. Another very important aspect is depth of field. To put it simply, depth of field is the area of your image that appears sharp around the point where you focused. You can change the size of this sharp area by adjusting the aperture. Keep in mind that, at the same subject distance:

In the context of this article, the larger the aperture (so the smaller the f-number), the shallower the depth of field. Conversely, the larger the f-number (for example f/14), the greater the depth of field. The two photos below were taken from exactly the same distance from the subject, with two different apertures. Look at the difference.

To sum up this point: if you open your diaphragm (a smaller f-number), you let more light reach your sensor, but you also reduce the depth of field. In other words, the whole image may no longer be sharp, especially across different planes in the scene. Depth of field is actually more complex than that, because it also depends on the distance to the subject and on the camera you are using. For the same framing and settings, depth of field is generally shallower on a full-frame camera than on a smaller sensor camera such as APS-C, Micro Four Thirds or a compact camera. Sensor size plays a real role here. A more detailed article will explain this point in greater depth.

We have already seen this together in the article on exposure in photography: aperture is directly linked to shutter speed and ISO. So what does that mean in practice? We have just seen that aperture already affects two things: the amount of light reaching the sensor and the depth of field. In addition to these two elements, aperture also directly influences shutter speed. How? Let me explain it simply. If you want to expose your image correctly and you are starting out in photography, you will probably choose “Av” mode (aperture priority). You set your ISO to automatic and your aperture to f/5.6, for example. The camera then gives you an appropriate shutter speed, let’s say 1/250. Now imagine that you want to close your aperture by one stop to f/8. What will the camera do? It will compensate for this loss of light by slowing the shutter speed down to let more light in. What does that mean? You had: ISO Auto – f/5.6 – 1/250. You now have: ISO Auto – f/8 – 1/125

To end this point, keep in mind that in normal use, for example in Av mode, if you want to keep your photo correctly exposed, the camera will compensate for your aperture change by speeding up or slowing down the shutter speed. It is then a matter of finding the right balance for the scene. The slower the shutter speed becomes, the harder it will be to freeze a scene.

In this last practical section, I’ll explain how controlling aperture can help you improve your photos, especially through depth of field. The main benefit of controlling depth of field is that you can isolate a subject from its background, or from its foreground. This is particularly useful for drawing attention to a detail in a subject or scene, for example a flower in a field, an animal in its environment, or a specific detail on a house compared with its surroundings. To do this, you open your diaphragm (a smaller f-number) and photograph your subject. This reduces the depth of field and helps separate your subject from the rest of the scene.

On the other hand, if you want the whole scene to be in focus, for example in landscape photography, you will need to close your aperture, meaning use a larger f-number. Be careful, though: as we saw above, the more you close your aperture, the slower the shutter speed becomes. You may then need either to increase the ISO to compensate or to use a tripod.

The creative possibilities are therefore endless. To practice and master aperture variations, as well as their impact on depth of field and shutter speed, I recommend the following exercise:

• Take your camera, go out in the field and start by setting aperture priority mode (“Av”). Set ISO to automatic (it’s easier at first) and choose the widest aperture of your lens, for example f/4 or f/5.6. You can then start taking photos of different subjects, in the shade, in the sun, close to the subject and farther away. Don’t hesitate to shoot as many photos as possible, since this is just a test;
• Write down or remember the photo number where you stopped;
• Repeat the same exercise with a smaller aperture this time, for example f/11 or f/14;
• Go home, look at your photos on the computer and start asking yourself a few questions.

Look at the photos that seem sharp and the ones that seem blurry, then check the associated settings for each one: ISO, aperture and shutter speed. Compare the photos taken with a large aperture and a small aperture, and observe which shutter speed was used. Was it fast enough or not? Is the photo actually blurry? By asking yourself these kinds of questions, you will gradually understand how aperture works and how it affects depth of field and shutter speed.

If you’re wondering when to choose a large or small aperture, here are the key points to remember from this article.

So, I hope you enjoyed this article and learned a little more about photography. Articles on the other two exposure settings, ISO sensitivity and shutter speed, will follow. There are still many concepts to understand before mastering everything, such as framing, hyperfocal distance, white balance and post-processing, but we’ll take it step by step!

See you soon for a new article.