Exposure triangle

To record a photograph that we can see, we have to control the amount of light that is exposed to the photosensitive surface, either film or a digital sensor – too much light and the image will be too bright with no detail, too little light and it will be too dark or black. The relationship between aperture, shutter speed and ISO is the cornerstone to understanding how to get the correct exposure. In this basic introduction, we will briefly explain these three elements and then discuss how they influence each other in the exposure triangle – there are links to more detailed explanations at the end of the article.

Aperture

The camera’s aperture is not part of the camera body, it is part of the lens. It’s the hole in the lens that opens and closes to allow more or less light in. It operates exactly like the iris of your eye. The larger the iris (lens diaphragm), the more light comes in. The smaller the iris, the less light. The aperture’s size is measured by a number called the F-number or F-stop. The larger the F-stop, the smaller the aperture – f/4 is smaller and lets less light in than f/2.8.

The aperture size can also be used creatively by controlling the depth of field of your image. Depth of field is the range of distance that appears to be focused in the picture. When focusing on a subject, the area in front of the subject is out of focus, along with the area behind the subject. The aperture controls how deep or shallow the area of focus is, for example a photograph of a person will often have a shallow depth of field to emphasise the subject while a landscape image may be sharp from the foreground all the way to the horizon. As a rule, the larger the aperture the more shallow the depth of field will be; with more areas of the image out of focus – this means that f/1.4 is going to give a much blurrier background than f/8.

Shutter speed

The shutter is the most basic component of a camera, it is essentially a curtain that covers your camera’s sensor that opens to allow light in, and closes to stop it. Adjustments in shutter speed simply changes the amount of time the sensor is exposed to the light therefore affecting the exposure. Shutter speed is measured in seconds and fractions of seconds.

Depending on the model of the camera, you may have the ability to shoot within the range of 1/8000th of a second down to a 30 second long exposure giving the option of controlling motion by freezing action or to blur moving objects in order to create a sense of motion. The bulb mode on your camera also allows for longer exposures with a shutter release cable, giving the ability to capture extreme long exposures. A shutter speed of 1/60 of a second or longer usually requires the use of a camera support or image stabilisation to prevent ‘camera shake’ – where the entire photograph is blurred as a result of camera movement.

ISO

ISO is simply a measure of how sensitive the camera sensor is to capturing light. As a rule, the lower the ISO, the better. As the ISO is increased the sensor will become more sensitive to other things in addition to the light you are trying to capture – this interference will manifest in your image as noise. The issue with noise is that it will cover up little details and makes the image appear soft. Additionally, when the noise gets really bad purple, white, and even green colors can start to appear in the black areas of the picture. The sensor is working so hard to capture light that it also records the electricity going through the sensor itself.

Exposure triangle factors

The exposure triangle is a useful way of describing the relationship between the three aspects of exposure. Each corner of the triangle represents one of the three variables: aperture, shutter speed and ISO. Adjusting just one of these will make the photo darker or brighter and will change the appearance of the photo based on what you have changed. For example, using a longer shutter speed will introduce motion blur to your photo but also make the photo brighter (increased exposure) due to more light hitting the sensor – to maintain the same exposure the ISO will need to be reduced or the aperture decreased.

While each of the three exposure variables (ISO, aperture, and shutter) work very differently, they all effectively produce the same result. They allow more or less light to reach the sensor or film. The key to creating stunning photographs is learning when would you change one variable over the other. “Correct” exposure is a semi-flexible term – there are no wrong settings, only different interpretations of the scene – one person may like dark and moody photographs while another prefers bright, airy images. In one sense, an exposure is correct if the image looks right. When I’m photographing people, the skin of their face is the most important factor.

A ‘technically correct’ exposure means avoiding muted or crushed blacks and blown-out whites while capturing as much photo data as possible in between.