The exposure triangle describes the relationship between aperture, shutter and ISO. The cause-and-effect nature of this relationship indicates that when you adjust one setting, another must change to create the same exposure. These three components work together to create harmony in a photograph. Watch this video for a simple explanation and demonstration of the exposure triangle.
ISO
International Standards Organization or ISO controls how much light a camera's sensor can capture. Film is sensitive to light. Too much light on the film overwhelms the film and produces pure white. Too little light produces dark and muddy results. This sensitivity is fixed, meaning it doesn't change over time. Once you know the exact amount of light required to expose film, it will never change. Sensitivity in photography is measured in ISO. ISO is a number. The greater the number, the better the camera sees in low light. Double the ISO, and the camera needs half the light.
A hundred years ago, film had an average rating of ISO 25. In those days, they didn't rate film stock, so people had to expose and develop by eye. A lot of light was needed to shoot anything. Slowly but surely, film came in higher sensitivities. Modern cameras start at about ISO 100, with some exceptions. Today, digital sensors have taken over. A sensor is like a piece of film that can change its sensitivity. The best sensors today can shoot cinematic images at ISO 3200 or 6400. Some cameras go beyond almost 4 million ISO and may go even further in the future. In the past, ISO was limited by the chemistry of film. Software can make changes to the sensitivity in some cameras. The sensor records the same thing, but through the software, you can change the sensitivity later. The limitation with ISO is actually in the opposite direction. Cameras go down to ISO 100, but some can't go below ISO 800. Some DSLRs go to ISO 50, but what if they could go lower? Today, we use neutral density filters to cut down light, but if we can reduce ISO to extremely low numbers, we won't need ND filters anymore.
Tip: Set your camera at auto ISO. You don't want to auto everything, but set your ISO to auto in the beginning. Why? The ISO is a functional tool. For the creative part of image making, you need the other two members of the triangle: the shutter and aperture.
Shutter
The shutter is a moving part of a camera that opens and closes, allowing light to pass through the lens and onto the camera's sensor. On cameras, it's written in seconds and minutes. In modern cameras, the shutter can stay open for hours or as low as 1/8000s. Having this range is powerful. If nothing moves while the shutter is open, the image is sharp. But if there is movement, like water flowing or clouds moving across the sky, these objects all get exposed while the shutter is open and blurs everything together. You can create stunning photographs this way. You can also capture faint stars by keeping the shutter open for hours. If you want to freeze droplets or you're shooting a fast-moving Formula One car, you can't keep the shutter open for too long. When things move during the time the shutter is open, the blur you get is called motion blur: blur caused by motion. This motion blur is undesirable in most photography. However, it is critically important in cinema. In cinema, there is a standard shutter duration cinematographers follow to this day. The shutter is kept at about 1/48s. This produces a motion blur that people have gotten used to over the last century of watching films.
Whenever you expose a photograph, that's one frame. In cinema, there are 24 frames a second. This means you have to open the shutter 24 times in one second to capture 24 frames per second. This also means you have to close the shutter 24 times a second. That's 48 total cycles per second, which is why the shutter speed is 1/48s. When Peter Jackson increased the frame rate of The Hobbit to 48 fps, the shutter would have been open and closed 96 times, for a shutter speed of 1/96s. This made the images crisper and reduced the motion blur. The reduced motion blur changed the impression of movement which is useful in sports. Sports are shot and broadcast at high frame rates so you can actually see the ball, for example, traveling fast. Otherwise, you'd see a blurry mess. You can change shutter speeds. If you want less light, keep your shutter open for a smaller period. If your ISO is low, you can open your shutter for longer to let more light through. ISO and shutter work like a balance. If you raise one, you lower the other for the same exposure. In film, the sensitivity is sometimes called speed, like film speed.
Tip: Think of the shutter as the inverse of the ISO. For example, if ISO 100 and a shutter of 1/100s give you one exposure, then changing the ISO to 50 will mean you must change the shutter to 1/50s. 1/50s is longer than 1/100s, so when you reduce the ISO by half, you also reduce the shutter speed by half.
Aperture
The aperture is the opening that controls the amount of light that enters the camera. The aperture is the mechanism that does this inside most lenses. Every lens is made of glass, and it has a fixed diameter. This diameter is the opening. The lens focuses the light and makes an image on the sensor, and the opening decides how much light can pass. The light that passes the opening must travel through the lens to the sensor. When light travels, it always loses intensity. The diameter of the opening and the distance it has to travel decide how much light hits the sensor. If you want to reduce the light, you can reduce the size of the opening. The aperture was designed because ISO didn't exist at the time the aperture was created. Something other than the shutter was needed to control light to work within the limitations of light.
If you open the aperture you let in more light, but you also get a smaller depth of field. Depth of field means the background gets more blurry which is useful for portraits or cinematic closeups. This is not ideal for landscapes or crowd scenes, where seeing everything is important. The aperture is critically important for macro shots. When you get closer, you need to make the aperture really small so the entire subject is in focus.
Tip: Think of the aperture and shutter as having two relationships. You can't raise both for the same exposure. Let's say you're standing in the open ground on a sunny day. You want a blurry shot of a flowing dress, but you want the background blurred. You have to open the aperture, and keeping the shutter open longer will cause too much light to come in. To solve this problem, use filters or lights, or reduce the ISO to compensate.
Numbers on Cameras
ISO and shutter are written in simple numbers. Aperture is written in odd numbers. The International Standards Organization has certain rules for how cameras should measure ISO. ISO 100 in one camera should give the same result as ISO 100 in another camera. Shutter speed is a mechanical thing controlled by time, like a wristwatch. 1/50s for one camera should be 1/50s for another camera. But with aperture, it's inside the lens. Every lens is different. Even when comparing the same lens made by two manufacturers, the optics inside are different. The diameter of the lens is different. The distance to the sensor is different. Both of these control the amount of light falling on the sensor. In other words, the aperture controls the exposure. Many aperture systems have existed in the past. In 1949, the world adopted the system we have today: the Uniform Scale. The Uniform Scale is f/1, 2, 4, 8, 16, 32, 64, 128.
Through trial and error, it was understood that the amount of light our world created by the sun is tremendous. You have to double or halve it to keep things under control. Every doubling of the light is called a stop, even though the number of the aperture is called f-numbers or f-stops. The reason why it's called the f-stop is because each major jump is the doubling of light or double the exposure. If you want a scale with 1, 2, 3 and so on, you can with f-numbers. F-numbers can be any number except zero. But with f-stops, each step is double or half the previous one, depending on the direction you're counting. F-stops are written as f slash number because on the lens you also have another linear scale: the focus scale. This is a way to differentiate between the two and to tell the f-stop is inverse; the larger the number, the smaller the aperture. In the early days of photography, Ansel Adams created the Zone system, with 10 stops. This was a latitude of film in those days, meaning you could expose on a sunny day and expect to see the range from bright to dark in 10 stops of light or 10 jumps on the scale. Today, many more stops exist. Ten jumps on this scale, starting from one and you'll see, if you use two as your base, the numbers get larger very quickly. Long ago, lenses were small, and numbers were etched on it. There wasn't much space, which is probably why root two was chosen: root two gives you smaller numbers. Each number represents a difference of one stop of light. When you close down the aperture by one stop, you halve the light.
Tip: Set your camera to full auto mode and point to a scene. Note the aperture, shutter and ISO settings. Then, turn the camera to manual mode. If the camera changes settings turn the aperture, shutter and ISO to the earlier settings. You should see the same image. Now, close down the aperture by one stop. For example, if it's f/2.8, change it to f/4. The scene becomes darker because reducing the aperture resulted in a loss of half the light. To compensate, either open the shutter up by one stop, the ISO by one stop or both in half stops.
Consider aperture as the setting that lets you create blurry backgrounds or make everything in focus. This is more practical and fun. Let the shutter and ISO change to balance the exposure. Keep one fixed and change the other two. Change both or change all three. These all marry together. Together, they form the exposure triangle, an elegant solution that has lasted hundreds of years.