Understanding Exposure Triangle: Aperture, Shutter Speed and ISO

exposure triangle

The exposure of a photograph is determined by three variables: Aperture, Shutter Speed and ISO, thus forming the exposure triangle. Each of these three variables plays their own role, a change in the value of one leads to a change in at least one of the other two variables. Thus it is really important to understand the functionality and role of each of the three variable of the exposure triangle.

Suppose your camera shows the correct exposure reading as f/4, 1/100 sec and ISO 100. If you increase the shutter speed by a stop (from 1/100 sec to 1/200 sec), you will have to reduce the aperture value (from f/4 to f/2.8) or increase the ISO sensitivity (from ISO 100 to ISO 200) by a stop in order to retain the same exposure. But what does the term ‘stop’ stands for?

What is a Stop and how does it affect the exposure of a photograph?

In photography terms, a Stop refers to doubling or halving the amount of light entering the camera in order to capture a photo. So if you hear someone saying that he/she has increased the exposure by 1 stop, it simply means that he/she has increased the amount of light entering the camera by twice. Similarly, if he/she decreases the exposure by 1 stop, it means that the light entering the camera has been reduced by half.

As the exposure of a photograph depends on aperture, shutter speed and ISO, an increase/decrease of a stop in one of these three variables would lead to decrease/increase of a stop in one of the other two variables.

Understanding Aperture

Aperture is one of the three elements of Exposure Triangle that are responsible for correctly exposing the photos. Aperture decides whether to create shallow or deep depth of field (more or less bokeh).

Aperture is the opening of the lens’s diaphragm made up of multiple blades which allow the light to enter the camera. The amount of light that enters through the lens depends upon the opening of the aperture (diaphragm). The wider the aperture opening, the more will be the amount of light that enter the camera. Similarly, the narrower the aperture, the less would be the amount of light that enters the camera.

aperture

aperture stops

The f-stop is inversely proportionate to the opening of the aperture.

  • The smaller the f-stop number = the bigger will be the aperture opening = more light will enter the camera
  • The higher the f-stop number = the smaller will be the aperture opening = less light will enter the camera

The smaller the f-stop, the shallower will be the depth of field which is good if you need to create blur effect in portraits and macro photography. Similarly, the higher the f-stop, the deeper would be the depth of field which can be used for landscape and cityscape photography to get the maximum area in focus.

To read about Aperture in-depth, visit: UNDERSTANDING APERTURE

Understanding Shutter Speed

Shutter Speed is one of the three elements of Exposure Triangle that are responsible for correctly exposing the photos. Shutter speed decides whether to freeze the subject in your frame or to capture a long exposure moment.

The shutter in the camera is a device used to allow light to enter and hit the image sensor or the film, after passing through the lens and the aperture diaphragm. Imagine the shutter to be a door, the moment you open it the light from outside enters the room and when you close it, the room becomes dark. The duration for which the shutter remains open is defined by the shutter speed.

Shutter Speed is the length of time for which the shutter is open to expose the image sensor/film. The shutter speed is denoted in seconds or fractions of a second, e.g. 2 sec, 10 sec, 1/50 sec, 1/250 sec, etc.

When you press the shutter release button on your camera, the shutter opens and exposes the image sensor to light for the duration of time defined. The faster the shutter speed is, the lesser the duration of time the image sensor will be exposed to light. Similarly, the slower the shutter speed is, the more the duration of time the image sensor will be exposed to light.

A faster shutter speed can be used to freeze the subject in your frame, whereas slow shutter speed can be used to capture the subject in motion or a long exposure shot.

To read about Shutter Speed in-depth, visit: UNDERSTANDING SHUTTER SPEED

Understanding ISO

ISO is the level of sensitivity of the image sensor inside your camera. Your camera allows you to choose ISO value ranging from 50 to 12,800 (or even higher) depending on the light conditions. ISO can affect your photo in two ways, it allows you to control the exposure and amount of grains/noise.

The lower the ISO number, the less sensitive the image sensor is to light. Thus, more light is required by the image sensor in order to properly expose the photo. On the other hand, the higher ISO number increases the sensitivity of the image sensor, therefore less light is required to properly expose the photo. As the sensor is less sensitive when you are using lower ISO number, the possibility of grains/noise appearing in your photos is very less. As you increase the ISO number and make the sensor more sensitive to the light, the grains/noise starts appearing in your photos.

Usually, the ISO number in a modern day camera starts from 50 or 100 and as you increase the value by rotating the wheel on your camera, the number doubles or halves with each click of the wheel. Suppose the base ISO of your camera is 100, as you move the wheel towards right (or any other key that increases the ISO number) the ISO value increases to 200. Similarly, if you move the wheel towards left it decreases to 50.

Understanding ISO Sensitivity in Digital Camera

With each step towards increasing the ISO value, the sensitivity of the sensor doubles and the exposure increases by 1-stop. With each step towards decreasing the ISO value, the sensitivity of the sensor halves and the exposure reduces by 1-stop. This is the reason the ISO sequence is such: 50, 100, 200, 400, 800, 1600 and so on.

To read about ISO in-depth, visit: UNDERSTANDING ISO SENSITIVITY

Conclusion of Exposure Triangle

Aperture: If you wish to keep everything in your scene in focus and sharp, you will have to select a high aperture value such as f/16 or f/22. But if you want the subject to be in focus and the background to be blur, then you will have to choose a small aperture value such as f/1.8 or f/2.8. Lower aperture value lets in more light, whereas higher aperture lets in less light.

Shutter Speed: If your motive is to capture the movement of a moving subject then you will have to choose a slower shutter speed such as 1/30 sec or slower. On the other hand, if you wish to freeze the movement of a moving subject then you would need to select a faster shutter speed such as 1/500 sec. Faster shutter speed lets in less light, whereas slower shutter speed lets in more light.

ISO: If the lighting condition is low and you do not want to slow down your shutter speed, then you can increase the ISO value in order to compensate. But after a certain ISO value, you will start noticing grains in your photos, this is when you should stop increase the ISO value by introducing an external light source or by using a tripod. Higher ISO value increases the image sensor sensitivity thus increasing the exposure and the noise, lower ISO value reduces the exposure as well as noise in the image.

About Author

Kunal Malhotra
Kunal Malhotra, a photography enthusiast whose passion for photography started 6 years back during his college days. Kunal is also a photography blogger, based out of Delhi. He loves sharing his knowledge about photography with fellow aspiring photographers by writing regular posts on his blog: The Photography Blogger. Some of his favourite genres of photography are Product, Street, Fitness and Architecture.