Aperture in photography: an indispensable notion

Recently, I’ve decided to start writing again on the basics of photography, which is I think a right resolution for my travel and photography blog. Some time ago, I published abasic article which should be read first by everyone wanting to improve, evolve and learn photography: exposure in photography. I invite you to read (or reread) it before starting this more technical and precise article. I’ll discuss here one of the three main parameters of the exposure: aperture in photography. For me, the aperture is the most delicate parameter to understand the notion of exposure. The two others are, in my opinion, easier to apprehend. Even if the notion may seem a bit abstract at first, once it is well defined and explained, and with a little practice, you will find the idea rather simple. I hope this article will help you understand how to take great pictures, according to which parameters, and that it will make it easier for you to get out of the automatic mode of your digital camera. If you bought a DSLR/mirrorless camera, it’s good to learn a bit of technical photography, isn’t it? Soon, the notions of shutter speed, long exposure, brightness and bokeh will no longer hold any secrets for you!

As explained in the mentioned above article, aperture is one of the 3 parameters that allows you to modify the final exposure of your image. The trio “Shutter speed / ISO sensitivity / Aperture” is essential to control your pictures. The final goal is (with the use of the two other parameters) to get good pictures by voluntarily controlling the amount of light that reaches your sensor. The key is to avoid (unless it is voluntarily desired) having an overexposed (too bright) or underexposed (too dark) picture. However, keep in mind that this is the most important parameter to give effects on your images (background or foreground blur effect, for example). We’ll talk about it again at the end of this article.

Let’s get straight to the point. In reality, instead of talking about aperture in photography, we should theoretically talk about opening the diaphragm. This represents “a piece” of your camera lens, made up of circular blades. The aperture is quite simply the diameter of this circle. This is mainly what you need to remember about the aperture.

Notions and notation

The aperture is given in “f/number”. Example: f/1.8, f/11 or f/13. The number behind the ” f/ ” corresponds to the value of your chosen aperture (size of the diameter of your diaphragm). This is called ” f/stop “. So far, it’s simple, isn’t it? But things get complicated as the notation of aperture (“f/”) is reversed in relation to the size of your diagram’s opening. Ok, now I think I’ve lost you, right? What you just have to understand is that the smaller the number behind the f/ (ex f/1.4), the bigger the aperture (your hole is bigger). And conversely, the larger the number behind the f/, the smaller the aperture (ex f/13). No need to go into technical details of why this quirk, just remember that:

Reading aperture on a lens

To explain it more concretely, on all the lenses you will buy, the maximum aperture of the lens is indicated (in addition to the focal length). It can be fixed (e.g. 70-200 mm f/4) or sliding (70-300 mm f/4-5.6). When you only have one number behind the “f/”, it means the aperture is constant no matter what focal length you have. If you have two numbers behind the “f/”, it simply means that your maximum aperture will vary depending on your focal length. To put it simply: the more you zoom on your lens, the less light you will get to the sensor of your camera. So, you’ll have a darker picture (which is annoying because you’ll have to compensate with one of the two other parameters).

The notion of light

The aperture in photography 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’s reversed!), the less light arrives to your sensor, and vice versa. You’ll hear about “closing the diaphragm” (going from f/8 to f/11 for example). In this case, you have closed one stop, and the amount of light has been halved. On the other hand, switching from f/5.6 to f/4 allows you to open your diaphragm and double the amount of light reaching the sensor of your camera. I’ll explain in the next paragraph how to change your aperture in little more details. For info, remember the 3 exposure settings are interrelated, and if you change the aperture, you will then change the exposure of your final image. To maintain the same exposure, you will have to change one of the 2 other parameters (ISO/Shutter speed). This is a very important notion to understand: the 3 parameters are all connected. You will therefore understand at the end of the article why a lens with a maximum aperture of f/2.8 costs twice as much as the same lens with a maximum aperture of f/4.

We have just seen the theoretical bases of the aperture and the associated notions. Now, let’s have a look at the practice and how we modify the aperture of the diaphragm. Here, no need to be a genius. You have two possibilities to set and modify your aperture. The manual mode: this is the most difficult mode to understand, and I don’t recommend it to beginners until you perfectly master the connection between shutter speed / ISO / aperture. Usually, you have to hold down the “Av/A” dial and turn the knob to adjust the aperture. The “Av” mode / Aperture priority: this is suitable for beginners and even for anyone else (I still use it a lot). The operation is simple, you set the aperture by turning the knob, then the ISOs (or you set to ISO Auto), and the camera will automatically calculate the shutter speed best suitable for the shot.

What to remember to close/open your diaphragm: I had already said a few words about it in the article about exposure, but I’ll remind it here. The system works with “stops”. Usually, current cameras offer intermediate settings, which often work by a third stop. Understand that the amount of light for each stop is doubled, let’s take the example of going from f/16 to f/11. It will be (often) necessary to turn the knob three times to go from f/16 to f/11, i.e. 3 thirds to make a stop. (f/16 to f/14 = 1 third then f/14 to f/13 = 1 third, then f/13 to f/11= 1 third). In this case, this doubles the amount of light.

As I mentioned earlier, the aperture of the diaphragm does not only influence the picture exposure. Another very important aspect is the notion of depth of field. To put it simply, it is the sharpest part of your photo, where you actually focus. You can change the extent of the sharpness area of the photo by playing with this aperture. Keep in mind that (at equal subject distance):

Regarding our article, the larger the aperture will be (thus the number f/ smaller), the more the depth of field will be reduced. Conversely, the larger the number (e.g. f/14), the larger the depth of field. The picture below was shot at the exact same distance from the subject, and with 2 different apertures. Gaze at the difference.

To sum up this point: if you open your diaphragm (one f/ smaller), you will bring more light towards your sensor, but you will also reduce the depth of field (the whole part of the image will not necessarily be sharp anymore, at least between the different levels). The depth of field is actually more complicated than that since it also depends on the distance from the subject during the shooting as well as on the camera. Be aware that the depth of field is “more pronounced” on a full-frame camera than on an APS-C, a mirrorless or a point-and-shoot camera. The size of your sensor will also play on all of it. A detailed article will explain this point deeper.

We have already seen it together in the article on exposure in photography; the aperture is directly related to shutter speed and ISO. So, what will it actually imply? We have just seen that aperture already affects 2 points: the quantity of light towards our sensor, but also the depth of field. In addition to these two elements, the aperture will also directly influence the shutter speed. How come? Let me explain it simply. If you want to well expose your image and you’re beginning in photography, you will choose the “Av” mode (aperture priority). You set your ISOs to automatic, and your aperture to f/5.6, for example. The camera will then show you an adequate shutter speed for shooting, let’s imagine 1/250th. Now, imagine that you want to close your aperture (f/larger) at f/8 (1 more stop), what will the camera do? It will compensate this loss of light by increasing the shutter speed (to let more light in). Which consequence? You had: ISO Auto – f/5.6 – 1/250. You now have: ISO Auto – f/8 – 1/125

To end on this point, keep in mind that in your normal use (“Av”, for example) and if you want to keep your picture correctly exposed, the camera will compensate your aperture change by increasing/decreasing the shutter speed. It will then be about finding a balance according to the scene. The more the exposure time decreases, the more delicate it will be to freeze a scene.

Last point on this article, I’ll explain you how controlling the aperture can help you improve your pictures and progress, especially about depth of field. The interest of controlling the depth of field is to be able to isolate a subject from its background (or its foreground, for that matter). This is particularly useful for details on a subject or a scene, for example: a flower in a field, an animal in its environment, a precise detail of a house compared to its surroundings, etc. To do this, you open your diaphragm (number f/small) and photograph the subject. This way, the depth of field will be reduced, and your subject will be isolated from the scene.

On the other hand, if you want to have a whole scene in focus (for example in landscape photography), you will have to close your aperture (so f/ larger number). Beware, as we have seen above: the more you close your aperture, the slower the shutter speed will be (and will require either an ISOs increase to compensate or the use of a tripod).

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

• Take your camera, go to the field and start by setting your aperture priority (“Av”), set the ISOs to automatic (it’s easier to start with) and choose the largest aperture of your lens (Ex: f/4 or f/5.6). You can now start taking pictures of various subjects, in the shade, in the sun, near or far from the subject. Don’t hesitate to shoot as many pictures as possible, since this is a test;
• Write down or keep in mind the number of the picture where you stopped;
• Repeat the same thing with a larger aperture this time (e.g. f/11 or f/14);
• Go home, look at your photos on the computer and start asking yourself some questions.

Look at the pictures that seem sharp and the ones seeming blurred and then look at the associated parameters (ISO / Aperture / Shutter speed) for each one. Check the differences between photos with a large and a small aperture and observe which shutter speed was used. Was it enough or not? Is the picture actually blurred? It is by asking yourself this kind of question that you will gradually understand how aperture works and how it impacts on depth of field and shutter speed.

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

So, I hope you enjoyed this article and you learned a little more about photography. Articles on the two other notions of ISO sensitivity and shutter speed will follow. There are still a lot of notions to understand in order to master everything (framing, hyperfocal, white balance, post-processing, etc.), but we’ll go easy on it!

See you soon for a new article,

Sylvain