Photography and geometry may, at first glance, seem like entirely different disciplines. But there is at least one geometric shape that is of great importance to photographers, and that is the triangle; more specifically, the Exposure Triangle.

So what is the Exposure Triangle? It is a way of describing how to get the correct exposure for every shot we take. At the bottom of the triangle, we have aperture and shutter speed. At the very top, we have ISO or film speed. These are the three fundamental controls that regulate the amount of light that will reach our camera’s sensor or film.

Understanding how these relate to each other is a vital part of understanding photography. Let’s have a little look at how the three work together, starting with the two most important elements of the triangle; aperture and shutter speed.

exposure in photography - exposure triangle


To describe the relationship between these two, we are going to use an analogy: a bucket of water. Imagine if you will, that the bucket is your sensor or film, and the water is your light. Our aim in getting the correct exposure is to fill the bucket to the brim without spilling any. If we under-fill the bucket, we are underexposing the image and it will look dark. If we overfill the bucket, we have overexposure and our image will look too light.

The secret to getting the correct exposure is to use shutter speed – how long the faucet or tap is open for – and aperture, which relates to how wide we open the tap. If we open the tap wide, but only for a very short time, we may not fill the bucket. For example, if we use a shutter speed of 1/2000th of a second (tap open for a short period) and an aperture of f4, (tap open fairly wide) and we find the bucket is not filling, we need to do one of two things: use a longer shutter speed or a wider aperture. Conversely, if we are over-filling the bucket, our options are to reduce the shutter speed or use a smaller aperture.

The way that these two main controls (aperture and shutter speed) relate to each other is one of the fundamentals of exposure in photography, because different shutter speed/aperture combinations will give you different-looking images in spite of the fact that the actual exposure value looks the same.

Shutter speed’s main effect on the image is in the way motion is conveyed. If you use a high shutter speed, say 1/1000th of a second, you will freeze fast-moving subjects such as racing cars or birds in flight. As you start to lower the shutter speed, moving subjects will start to show motion blur. Landscape photographers are particularly fond of slow shutter speeds. This significantly blurs movement such as water in a waterfall or seascape, giving us those beautiful, ethereal-looking landscapes we often see. Of course, since exposure is a balancing act, the photographer has to compensate for the slow shutter speed by using a small aperture.

exposure in photography - moving vehicles on a streetA slow shutter speed will give motion blur

exposure in photography - penguinsA fast shutter speed will freeze motion

The main effect of aperture is depth of field; or in other words the amount of the image that is in reasonable focus in front of and behind the main focus point. If we use a wide aperture, we get a shallow depth of field, meaning the image will be out of focus in front of and behind the subject. This is often used by portrait photographers to bring the viewer’s attention to the subject. Conversely; if we use a small aperture, we get a deep depth of field, often from the foreground all the way to the horizon when used in landscape photography. Again, we have to balance our aperture selection by using the correct shutter speed.

exposure in photography - flower with a blurred backgroundA wide aperture gives shallow depth of field

exposure in photography - boat on icy watersA small aperture gives deep depth of field

The last point on the triangle is the ISO, or film speed. In the days of film, this was set as soon as you closed the camera back and couldn’t be changed. However, digital photography allows us a wide range of ISO settings with which to control our exposure. ISO in general should be seen as the last option on the exposure triangle; the option to use if we cannot get the right exposure with shutter speed and/or aperture.

Typically, ISO comes into play when the light levels are low and we cannot achieve a high enough shutter speed to freeze the action or eliminate camera shake. By increasing the ISO, what we are doing is effectively increasing the sensitivity of our sensor to light. To use our analogy, we are making the bucket smaller, so we need the tap on for a shorter period of time. The major downside to this is that any increase in ISO causes extra noise in our images – noise showing up as graininess in the shot. Today’s digital cameras can control this pretty well up to a point, but for the most part, the best option is to use the lowest ISO that you can get away with given the lighting conditions.

exposure in photography - ferry on Hongkong bayHigh ISO can introduce noise

Understanding the exposure triangle will help you understand why your images look the way they do. Learning the relationships between shutter speed, aperture, and ISO will allow you to become a master of the light, using your camera to much greater creative effect.

About The Author

Jason Row

Jason Row is a British born travel photographer now living in Odessa Ukraine. His work has been published worldwide in newspapers, books magazines and strangely on towels from a Turkish textile company.

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