The Exposure Triangle: ISO, Aperture, and Shutter Speed

Almost all of the new cameras come with advanced auto modes that basically does all the work for you. It will set the aperture, shutter speed and ISO for the shot, and all you have to do is point and click. However, using the auto mode puts many limitations on what you can achieve with your camera. So, moving out of the auto mode should be a good idea!

What is this Exposure Triangle? The ‘Exposure Triangle’ may sound like a sci-fi movie, but it actually is a term used to collectively refer to the three fundamental variables that determine the exposure of a photo: ISO, Shutter Speed, and Aperture. They are also known as the ‘Three Pillars of Photography’ since they not only affect the exposure, but also other factors on which the overall appearance of the photo depends. Having a good understanding of the exposure triangle will allow you to take control over your images, instead of the camera controlling them.

Understanding ISO, Shutter Speed & Aperture

ISO is an acronym for ‘International Standards Organisation’. It is a measure of the light sensitivity of the image sensor. ISO ranges from 100 to more than 64,000. Higher the number, more will be the sensitivity of the sensor. Using high ISO lets you use high shutter speeds in low light conditions, but it also introduces noise or grain in the image. Therefore it is recommended to always use low ISO to avoid noise, and use high ISO only when really needed.

Shutter Speed
A shutter is a flap that covers the image sensor, which moves down and exposes the sensor for a pre-set period of time when we click a photo. The camera will continue to record an image for as long as the shutter stays open. Hence it is also known as the exposure time. Shutter speed basically means how fast your camera clicks a photo. High shutter speeds are used to freeze fast moving subjects, while slower speeds are used to show motion. A longer/slower shutter speed also means that the sensor will be exposed for a longer time, collecting more amount of light. If you are shooting in low light conditions, you will have to use slower speeds to allow the sensor time to get ample light. Similarly if you are out in daylight, you will have to use faster speeds to avoid overexposed images.

This is perhaps the most difficult to understand for most people, but easy to use once you do. The aperture refers to the opening of the lens, which controls the amount of light reaching the image sensor. Apart from the exposure, it also controls the depth of field of the image, which basically means how much area of the scene is in focus. Aperture values are denoted in ‘f-stops’ and usually range from f/1.4 to f/22. A smaller number means a bigger opening, which in turn means more light and a shallow depth of field.

Filling a bucket of water!

This is related to the article, I promise. It is actually an example which will help you easily understand the concept of ISO, aperture and shutter speed, and the relation between them. Let’s imagine you are at your house and you want to fill a bucket with water for bathing. So you go the bathroom, place the bucket under the tap and open it. But there is more than one way to fill the bucket. You can either open the tap fully which will fill the bucket quickly, or you can open it slightly in which case the bucket will take more time to fill. Think of water as the amount of light you require to correctly expose a photo. The tap is the camera aperture which opens up wide, or not so wide, to let light into the camera. The amount of time it takes to completely fill the bucket is the shutter speed. If you choose a wide aperture, you need to keep the shutter open for a short time. But if you choose a small aperture, you will have to open the shutter for a longer time to ensure that you capture enough light. Now the size of the bucket is corresponds to the ISO level. A higher ISO means a smaller bucket, meaning if you increase the ISO then it will take less time to fill the smaller bucket.

The Relationship!

The exposure triangle is called a triangle because each of the three elements affects the other two. The shutter speed, aperture, and ISO each affects the exposure of the image. When these three elements are combined, they represent a given exposure value for a given setting. Changing any one of the settings will require a change in at least one of the other two, to maintain the exposure. You must balance all three of these to achieve a desired result.

Getting the right exposure is a juggling act because it requires the right balance of ISO, aperture and shutter speed. The camera can do this for you in the auto or semi-auto modes, but everything depends on you in the manual mode. One simple trick to follow is that when you increase the exposure for one element, you need to reduce it for one or both of the other elements in order to maintain the same exposure.

In addition to their role in exposure, the choice of aperture, shutter speed and ISO have a significant impact on the look and feel of your pictures. Higher ISO will let you shoot in low light but results in noisy pictures. Slower shutter speeds increases the amount of light reaching the sensor, but can also cause blurring. A smaller aperture will let you get a larger area in focus, but the smaller opening results in less light reaching the camera. It is impossible to change one of the elements and not obtain an opposite effect in how the other elements affect the image, and ultimately change the exposure. You have to decide which aspects you want in your photo and what you can do without, all while maintaining the required exposure.

Making sense out of the confusion!

Now I know this is all very confusing, with everything having an effect on everything else. Let me try to make this easier for you to understand by taking an example. Let’s imagine that you have gone to the beach in the evening to capture the beautiful scene of a sunset. The task in front of you is to select the appropriate settings of the ISO, aperture and shutter speed such that it results in a correctly exposed photo, while maintaining the feel and quality.

The first thing you will consider is the ISO. The sun has almost settled and there is not much light at the scene. You know that selecting a high ISO level would help deal with the low light, but then that would result in a very noisy picture. And you don’t want noise to disrupt the beauty of the scene. So you select the lowest ISO on your camera, say 100.

Then you come to aperture. Since it is a landscape shot with the sun being the subject, you want the foreground as well as the background to be in focus. To do that you set the aperture at say f/11. But the small aperture results in less light reaching the sensor.

Then you reach the final element of shutter speed. In order to compensate for the low ISO and small aperture, you will have to use a slow shutter speed to allow ample light to reach the sensor. So you set up the camera on a tripod to avoid any blurring because of the slow speed.

So you have set the exposure triangle, and you click the photo. Wow what a beautiful shot! The image is free from noise, everything is in focus, and it is properly exposed. But let us imagine that you forgot to bring your tripod to the scene. Without a tripod you are forced to use a fast shutter speed otherwise the photo will come out blurred. But doing so will also result in the image getting underexposed. To deal with the problem, the best option would be to increase the ISO to allow the use a faster shutter while maintaining the correct exposure. Yes the image would be a bit noisy, but it is acceptable compared to a blurred photo.

With this example you can easily understand the process of how to select the appropriate settings. Different situations will call for different actions, and it depends on you what is most important for the shot, and what you can compromise.

Do not expect to know every time exactly what settings to use, you will have to find out the perfect combination through trial and error. Keep in mind that changing one element not only impacts the exposure of the image but also has an impact upon other aspects of it. Mastery over the three elements of exposure is crucial for both technique and composition. But this mastery can only come be achieved with a lot of practice. The advantage to digital photography is that it costs nothing to experiment with your camera, so go out there and shoot away. You want to become proficient with all three elements of the exposure triangle, so that you can make adjustments on the fly and know exactly what the result is going to be.