Exposure Triangle: What, Why And How

Exposure TrianglePrelude

One of the most important concepts of photography involves three critical functions of a DSLR. Most DSLR users when starting out leave their camera on the Auto mode, which is convenient but not advisable. If you are only going to use your DSLR like a Point & Shoot camera it would make a lot more sense to actually buy one and save yourself some money. The reason you bought a DSLR is because you wanted to have more creative control over what you shoot. In order to do that you need to move out of the Auto mode and into one of the manual modes. Get some pointers to get started with the manual modes on your DSLR. Join this great course.

The concept of exposure triangle discussed here is applicable to all DSLR cameras ever produced. They also apply to some advanced compact cameras as well as to Mirrorless Interchangeable Lens Cameras (MILC) where you can tinker with the shooting modes and change Aperture, Shutter speed and ISO. That was a giveaway.


Exposure Triangle refers to Aperture, Shutter Speed and ISO of a camera and their mutual relation. If your camera comes with a shooting dial with the letters M, A & S then this is applicable to you. The main thing that you need to understand is that these three aspects control the amount of light that is captured by your camera. Changing one invariably affects the other two. It means if you change one of these you will need to change the other two as well in order to get the correct exposure. Understand the basics of a correct exposure by joining this course.


Aperture is the opening of the lens. Light travels through this and hits the sensor, where it is collected, processed and encoded. Aperture size can be controlled, enabling the camera to collect more or less light depending on the situation. In this sense the aperture functions almost the same way as the pupil of the human eye does. When you step outside in the sun your eyes automatically adjusts the size of the pupil so that less light passes through. When you step in to shade or inside a dimly lit room they dilate so that more light can pass through.

Aperture is expressed in f-numbers. It looks more like a fraction. You may have seen such expressions as f/8, f/11 etc. in relation to aperture. This is nothing but an indication of the size of the aperture. When the denominator is bigger aperture size decreases. The reverse happens when the denominator is a smaller number. This means f/2.8 denotes a bigger aperture compared to f/8. This can be confusing to grasp initially, but as you progress further into photography this will become much easier to remember and hopefully second nature to you.

Shutter Speed

Though this is referred to as Shutter ‘Speed’, in reality it is more to do with time. In fact Canon’s mode dial refers to it as Tv (Time Value). Shutter Speed denotes the length of time for which the lens remains open to collect light. Evidently the longer it remains open the more light it can collect. Here is an online course that discusses in detail the creative uses of long shutter speed.

Shutter speed is expressed as a fraction too, such as 1/8, 1/15, 1/30, 1/60, 1/125 and so on. The smaller the fraction the faster is the shutter speed. In other words less is the amount of time that the lens remain open. To set shutter speed you need to take your camera out of the P mode (Program mode) and set it on either Shutter Priority or Full Manual mode (M). Turn the main mode dial till ‘S’ or ‘Tv’ or ‘M’ is selected. Now depending on the camera type and make you can either turn the main wheel or go to the main menu and select the shutter speed.

While the aperture is automatically calculated by the camera in shutter priority mode, you will need to manually select both shutter speed and aperture in manual mode.

Shutter Speed and Aperture shares an inverse relationship between them. Meaning, if you select a bigger aperture (smaller f-number), in order to maintain a correct exposure the shutter speed has to be increased.


ISO stands for International Organization for Standardization. In relation with photography ISO refers to the digital sensor’s sensitivity to light. ISO is expressed in numbers such as 100, 200, 400, 800 and so on. Sensitivity doubles as you move progressively from one number to the other towards right and halves if you move leftwards. The minimum that most digital sensors can go is 100, though there are some Point & Shoot cameras where the minimum ISO can go to as low as 80.

Why & How

Let’s look at some practical uses of this relationship between shutter speed, aperture and ISO.

Exposure Compensation

When you look through the viewfinder in either of the priority modes (aperture priority or shutter priority) or the Program mode you will notice that there is an exposure meter. It basically gives you an indication of the exposure. If it is on ‘0’ that means you have the right exposure and nothing needs to be done (hypothetically). However, you can change that in a situation where you want to suppress the highlights or increase saturation and bring more details out from the shadows.

Different cameras have different mechanisms for using the exposure compensation mode. Let’s say you have one of the Canon Rebel Series cameras. There is an exposure compensation button next to the LCD screen on the back of the camera. It is the button with the symbols (+/-). Press that and immediately the exposure meter lights up in the viewfinder. Now, when you turn the dial at the front of the camera you will notice that the exposure compensation meter indicator slides left or right depending on which way you are turning the dial.

Exposure compensation can be used in aperture priority, shutter priority and program mode. In shutter priority mode this will adjust the aperture value to adjust the exposure. This is however, not recommended. If you really want to use the shutter priority mode and not sure about the aperture value to use, it is recommended that you use the manual mode instead. In aperture priority mode Exposure compensation will change the shutter speed and in Program mode it will adjust both the aperture value and the shutter speed to get the correct exposure. Exposure compensation cannot be used in manual mode. At least not the same as you do in the other modes. In manual mode the camera’s exposure is completely manually controlled. That means you decide which shutter speed and aperture value to use.

Exposure Value

Exposure value is any combination of the shutter speed and aperture that yields the same exposure for a given scene under a given lighting condition. Thus, one pre-requisite for exposure value to work is that the lighting condition should not change and neither should the scene. Let’s say in Auto mode the camera meters a scene at f/5.6 and 1/125 of a second. If you decide that at f/5.6 you are not going to get a nice bokeh you can open up the aperture to f/2.8. But that means a two stop increase in the Aperture! In order to compensate for the additional light coming through now, you have to shorten the time frame for which the lens remain open, i.e.; use a faster shutter speed. In this case a two stop faster shutter speed would be 1/500 of a second. Thus, theoretically, 1/125th of a second at f/5.6 will yield the same exposure as 1/500th of a second at f/2.8.

Tweaking the ISO

Tweaking the ISO does almost the same thing as tinkering with the shutter speed. E.g., when you are in a low light condition setting ISO 1600 has the same effect of reducing the shutter speed and collecting more light. However, higher ISO numbers should be used with discretion because they tend to amplify the noise that is associated with digital files. Higher the ISO you set more is the noise associated. On the plus side, however, you can avoid image blur because you don’t need a really long shutter speed in order to capture all the light you need for a proper exposure. It also saves you from hauling a few extra pounds in the form of a tripod.

The Sunny 16 Rule

To conclude let’s quickly discuss about another concept and this involves all three aspects of exposure triangle. This concept was devised many years ago but is still valid and produces great results every time.

Let’s say it is a bright sunny day and you are shooting outdoors. What aperture value or shutter speed and for that matter ISO to use? The Sunny 16 Rule states that when you are out in the sun in a LV15 or may be even 16-ish brightness, set your aperture to f/16. Let’s say you decide to set a very low ISO (recommended) of 100. Your shutter speed should be the inverse of your ISO, i.e., in this case 1/100th of a second. If you choose a slightly higher ISO of 200, then your shutter speed should be 1/200th of a second.

Want to know more on how to use the above concepts and tips in your photography? Join this great course on mastering digital exposure.