Exposure triangle – Aperture
Welcome to part two of our exposure triangle article. In our previous article we discussed ISO and how changing the value can change the sensitivity of the sensor. In this article we discuss the next parameter in the exposure triangle APERTURE.
Let’s start with the definition of aperture. APERTURE is the unit by which we measure how open a lens is. Just in case this doesn’t make sense, let’s go into a little bit of detail on how a lens works. Now, we’re not going to be talking about how a lens focuses, instead, we are going to concentrate on how a lens allows light to pass through it into the camera.
The amount of light the lens allows to go through it is not always the same. inside the lens there is a mechanism which allow the lens to close down and open up much like the pupil in your eye. When you walk in from a dark environment into a bright environment your pupil shrinks to let in less light. The same thing happens when you walk in from a bright environment into a dark environment your pupil will open up to allow more light into your eye. A lens works in a very similar way. Inside the lens there is something called a diaphragm . This diaphragm is and mechanical component of the lens that acts exactly like the pupil in your eye. It can close down to allow less light to come in, or it can open up to allow more light come in.
The opening of this diaphragm is referred to as the APERTURE. Sometimes, this is also referred to as the F-stop. In order to make this article easy to follow, We will refer to aperture and F-stop as the same thing. The F-stop numbers IRA strange bunch of numbers. You may have seen them before. F-2.8, F-4, F-11, etc.
This next bit is not really that important but just in case you’re interested in learning what those numbers actually mean, here it is. The numbers simply refer to a fraction. let’s say for example that a lens was able to open up all the way . That is, the diaphragm would not be visible anywhere. This would be considered F-1. Now let’s imagine that the diaphragm closed down to only allow for half of the light to come through the lens. This would be considered F-2. if the lens closed down to allow for only 1/10th of the light to come through, this would be considered F-10. As you can see, the number is simply a fraction.
The first most obvious result of opening up or closing the lens would be the amount of light that comes in through the light. This is correct. In darker scenes, one could open up the lens to something like F-2.8 and allow more light to come into the camera. The same approach can be taken if you are somewhere that is extremely bright and you want to reduce the amount of light coming into the camera. In this case you could close the lens to something like F-16. So far all this seems to make sense and seems like a pretty easy way to control the light that comes into the camera, but there is another effect that is caused by opening up or closing the lens and that is called depth of field. When you are focusing your camera onto your subject ,typically everything in front of that subject or behind that subject is going to be out of focus. But exactly how big is this area of focus? Let’s take a portrait for example. Let’s say that you focus on somebody’s eye. Will the tip of their nose be out of focus or in focus? This is where the F-stop comes into the picture. One of the characteristics of opening up the lens to something like F-2.8 results in a very small depth of field. So depending on how far away you are from your subject if you focused on the tip of their nose their ears might be out of focus. In this case you would have to stop down your lens which simply means racing your F-stop to something like F-5.6 to increase the depth of field. In summary, the larger the aperture the smaller the depth of field and the smaller the aperture , the larger the depth of field. This effect is amplified when the camera is closer to the subject, and it is lessened as you move further away from your subject.
This is the reason why often when photographers are taking pictures of groups of people they will use an F-stop around F-8 or F-11 to allow you to photograph a group of people and have the people at the front of the group and at the back of the group all be in focus. Of course closing down the lens results in letting less light into the camera which means you may have to compensate by using the ISO or the shutter speed. Of course there are times when having a blurry background looks fantastic. When taking portraits for example this is a really good way to separate your subject from the background.
Below is a chart that summarises and describes the effect of increasing or decreasing your aperture.
Infographic explaining depth of field and the corresponding aperture values with their effect on blur and light
It might all sound a little bit confusing in the beginning, but the more and more that you practise it will become second nature to you. Of course you can have the camera automatically set the F-stop for you. But if you do this you’re handing over some of the creative power back to the camera. in our previous article we discussed ISO and how that affects the sensitivity of the sensor do either become more light sensitive or less light sensitive. Using your aperture along with your ISO is a great way to find a compromise in order to get the photograph that you’re after. In our next article we will discuss the shutter speed. This is the final component of the exposure triangle that we will use to craft our photograph.