How does aperture work?

What does aperture mean?

There is a lot to tell about aperture in photography, but essentially it is just a hole. An opening. An inlet. The aperture is the hole through which the light enters your camera before it falls onto your sensor. The cool thing is, that you can change the size of the hole using a mechanism called the iris. And it’s no coincidence that it’s called that way, because the mechanism works about the same as the iris in your eye. By contracting, the iris closes the hole down, thus letting in less light. The other way around, it opens the hole up to allow more light to enter.

So much for the meaning of the word. But what does aperture really mean in relation to photography? Because it’s more then just a hole. By controlling, among other things, the amount of light that enters the camera, the aperture is an essential part of your camera. It is one of the three key parts making up the exposure triangle. In this article I will try to explain what I have learned about the aperture, and how it influences your photography.

The technical bit

The part inside your lens that controls the aperture is the iris. This consists of a number of thin blades that slide over each other from the sides of the lens to create a hole in the center, the aperture. You can see what this looks like in the picture below. The more blades in you iris, the closer the aperture comes to forming a perfect circle. Typically more expensive lenses have more aperture blades. The shape of your aperture, also defines the shape of the bokeh you can find in some images.

The f-number

The size of your aperture is referred to with a number, which is called an f-number. So when you’re setting your camera up to take a photograph you can control the size of the aperture by changing the f-number. The interesting thing is that choosing a lower number opens the aperture up, creating a larger hole (and letting in more light). Off course a higher number then means closing the hole down (and letting in less light). This might be a bit confusing, but it can be explained by the way the aperture size is calculated.

The f-number, or often called f-stop, does not represent the physical size of the aperture. It represents the amount of light falling onto the sensor. And this is not only related to the aperture size, but also to the focal length used. When you increase the focal length, a large aperture is needed to get the same amount of light to fall onto the sensor. So to counter that effect, the same f-stop number will always expose the sensor to the same amount of light, even when the focal length changes. This is where it gets mathematical.

The maths

To calculate the f-number the following formula is used. And using the mathematical rules we learned in high school, we can also turn that around to a formula that calculates the diameter of the aperture at a given f-number and focal length.

That second equation is actually what you see on most cameras when it comes to selection your aperture value. When it says f/2 on your camera, the “f” stands for focal length and the 2 stand for the f-number. So what it says is: “the aperture diameter is the current focal length divided by 2”. Changing your focal length would result in a change in the aperture diameter, but will not automatically affect your f-number.

Aperture steps

And then we’re not done with the math just yet. When photographing, you set the aperture in steps. Usually this is done in the camera settings, but sometimes it’s done on the lens. For these steps there is a scale. Depending on the lens you’re using, these start somewhere around f/1 which would be your largest aperture, and can go up to f/44 which would be the smallest.

There are some exceptions, but the base scale follows a pattern. Every next step is a multiplication of √2 of the previous step. This also means that every second step is a doubling. So the third number is double the first, the fifth is double the third and so on. Same goes for the fourth being double the second, and so on. Check out the schematics below, that shows the basic scale. As said, there can be exceptions to this for your lens/camera combination.

Aperture in different types of lenses

There are a couple of factors that can influence the available aperture settings for your lens. Typically, more expensive lenses can have greater apertures. Also prime lenses (having only one fixed focal length) can handle greater apertures then zoom lenses. Finally, usually lenses with greater focal lengths normally have smaller maximum apertures (which means a greater number).

The effect of changing your aperture

That’s all nice to know, but the big question is: what effect does changing the aperture mean for your photo? Before we can answer that question, we need to understand what the aperture does. There is two main things your aperture controls:

The amount of light that reaches your sensor
The depth of field

I would consider the first of these two to be mostly a technical consideration, and the second to be mostly a creative one. Let me explain that a bit more.

Aperture letting in light

When you take a photo, you want it to be technically sound. That means you want it be sharp, focused in the right area and properly exposed. One of the main factors in photography is the correct exposure, which means you have to control the light coming in.

When you find yourself in a situation where there is not so much light, one of the options you have to reach a correct exposure, is to open up the aperture. That way you are taking in more of the available light and might be getting better results in your pictures. Letting in more light through the aperture, allows for a faster shutter speed and/or a lower ISO setting. Simple as that.

Off course, this also works the other way around. When you are in a very bright surrounding where too much light is coming in, you can choose to close down the aperture. That way, you will block some of the light from entering your camera so your photos don’t overexpose.

Aperture controlling depth of field

The other thing you want for your photos, is for them to be pleasing to look at. That’s where the creativity comes in. One of the creative choices you can make is the depth of field in you photo. This means the area of your photo that is in focus, as opposed to the part not being in focus. When using a small aperture, more of your photo is in focus. Two other factors also contribute to this, these are the distance to your subject (focal point) and the focal length you use. When your subject is further away, your depth of field (at the same aperture) will be greater. The same goes for using a shorter focal length.

You can see an example of this in the series of photos below. The photos from left to right were shot at an increasingly smaller aperture. In the far left picture, shot at f/2, you can see there is only a small depth of field. Only the figure I focused on is on focus, the figures in front and behind her are out of focus. By closing down the aperture, the other figures come more into focus. At f/16 all four are almost completely in focus. Only at f/22 they are all completely in focus and sharp.

Using aperture creatively

You can use this knowledge to your advantage when you are taking your photos. If you wan’t a specific subject to stand out in your photo, you can use a larger aperture, like the f/2.0 in the example. This way the subject that you focused on will be sharp and everything around it blurry. By doing that, the attention of the viewer is drawn to the in focus subject immediately. In landscape photography it’s more common to use a smaller aperture like f/9.0, so that the landscape you photograph will be sharp from the foreground to the background.

Aperture in your eyes

It is is more or less the same with your own eyes. The cool thing about this, is you can test this theory yourself. If you look at an object an object that is very close to your eyes, and you focus on it, you will only see a small part of the object sharply. If you focus on something in the distance, the object that is close to you will be blurry (out of focus). Finally if you stand in an open field and you look in the distance, almost everything appears to be in focus.

Depth of field - aperture vs focal length

As I explained in the boring math part earlier in this article, the aperture size inside your lens changes when choosing a different focal length. This also means that the depth of field is changed along with it. Please keep in mind that although the physical size of the aperture hole changes, the aperture number in your camera setting will remain unchanged.

So how will these changes manifest themselves when taking photos? I will try to explain using a couple of examples.

Depth of field at close range

So my first example is a situation with a relatively close focal point. Imagine shooting a model at a distance of 2 meter. Let’s use a nice 24-70mm lens for this at aperture f/9.0. If we take three photos at different focal lengths this lens supports, depth of field will be as follows:

Focal point	Aperture	Focal length	Depth of field
2 meter	f/9.0	24mm	26.62 meter
2 meter	f/9.0	40mm	1.49 meter
2 meter	f/9.0	70mm	0.43 meter

As you can see, the depth of field gets a lot narrower at a longer focal length, even when the aperture stay put at f/9.0.

Depth of field at long range

Let’s do the same thing, but change the scenery a bit. Now we will go for a bird sitting in a tree at 100 meters distance. For this we will use a 100-400mm lens, to bring the subject a bit closer. To let in a bit more light, the aperture will be at maximum opening for this lens at f/5.6. Now the depth of field will look like this at different focal lengths:

Focal point	Aperture	Focal length	Depth of field
100 meter	f/5.6	100mm	infnite
100 meter	f/5.6	250mm	57.88 meter
100 meter	f/5.6	400mm	21.18 meter

Again, you can see the depth of field gets narrower as the focal length increases. An other thing you may notice, in comparison to the close range situation, there is a reasonably large depth of field still left even at the largest aperture and longest focal length. To make these calculations, I used the very useful app Photographer’s companion. Download it here:

Conclusion

As you can see, the aperture is an important part of photography. Both in technical and a creative sense it has a big impact on the outcome of your photos. The best way to learn how to use the power of the aperture, is to practise with it and see what the effect of changing the setting is. So go out there and start shooting!