How Do Shutter Speed, Aperture and ISO Work Together to Create an Exposure?

To have a good understanding about exposure and how shutter speed, aperture and ISO affect it, we need to understand what happens within the camera when a picture is taken.

As you point your camera at a subject and press the shutter button, the subject gets into your camera lens in a form of light. If your subject is well-lit, there is plenty of light that travels into the lens, whereas if you are taking a picture in a dim environment, there is not much light that travels into the lens. When the light enters the lens, it passes through various optical elements made of glass, then goes through the lens “Aperture” (a hole inside the lens that can be changed from small to large). Once the light goes past the lens aperture, it then hits the shutter curtain, which is like a window that is closed at all times, but opens when needed. The shutter then opens in a matter of milliseconds, letting the light hit the camera sensor for a specified amount of time. This specified amount of time is called “Shutter Speed” and it can be extremely short (up to 1/8000th of a second) or long (up to 30 seconds). The sensor then gathers the light, and your “ISO” brightens the image if necessary (again, making grain and image quality problems more visible). Then the shutter closes and the light is completely blocked from reaching the camera sensor.

To get the image properly exposed, so that it is not too bright or too dark, Shutter Speed, Aperture and ISO need to play together. When lots of light enters the lens (let’s say it is broad daylight with plenty of sunlight), what happens when the lens aperture/hole is very small? Lots of light gets blocked. This means that the camera sensor would need more time to collect the light. What needs to happen for the sensor to collect the right amount of light? That’s right, the shutter needs to stay open longer. So, with a very small lens aperture, we would need more time, i.e. longer shutter speed for the sensor to gather enough light to produce a properly exposed image.

Now what would happen if the lens aperture/hole was very big? Obviously, a lot more light would hit the sensor, so we would need a much shorter shutter speed for the image to get properly exposed. If the shutter speed is too low, the sensor would get a lot more light than it needs and the light would start “burning” or “overexposing” the image, just like magnifying glass starts burning paper on a sunny day. The overexposed area of the image will look very bright or pure white. In contrast, if the shutter speed is way too high, then the sensor is not able to gather enough light and the image would appear “underexposed” or too dark.

Let’s do a real-life example. Grab your camera and set your camera mode to “Aperture Priority“. Set your lens aperture on your camera to the lowest possible number the lens will allow, such as f/1.4 if you have a fast lens or f/3.5 on slower lenses. Set your ISO to 200 and make sure that “Auto ISO” is turned off. Now point your camera at an object that is NOT a light source (for example a picture on the wall) then half-press the shutter button to acquire correct focus and let the camera determine the optimal exposure settings. Do not move your camera and keep pointing at the same subject! If you look inside the camera viewfinder now or on the back LCD, you should see several numbers. One of the numbers will show your aperture, which should be the same number as what you set your aperture to, then it should show your shutter speed, which should be a number such as “125” (means 1/125th of a second) and “200”, which is your sensor ISO.

Write down these numbers on a piece of paper and then take a picture. When the picture comes up on the rear LCD of your camera, it should be properly exposed. It might be very blurry, but it should be properly exposed, which means not too bright or too dark. Let’s say the settings you wrote down are 3.5 (aperture), 125 (shutter speed) and 200 (ISO). Now change your camera mode to “Manual Mode“. Manually set your aperture to the same number as you wrote down, which should be the lowest number your camera lens will allow (in our example it is 3.5). Then set your shutter speed to the number you wrote down (in our example it is 125) and keep your ISO the same – 200. Make sure your lighting conditions in the room stay the same. Point at the same subject and take another picture. Your results should look very similar to the picture you took earlier, except this time, you are manually setting your camera shutter speed, instead of letting your camera make the guess. Now, let’s block the amount of light that is passing through the lens by increasing the aperture and see what happens. Increase your aperture to a larger number such as “8.0” and keep the rest of the settings the same. Point at the same subject and take another picture. What happened? Your image is too dark or underexposed now! Why did this happen? Because you blocked a portion of the light that hits the sensor and did not change the shutter speed. Because of this, the camera sensor did not have enough time to gather the light and therefore the image is underexposed. Had you decreased the shutter speed to a smaller number, this would not have happened. Understand the relationship?

Now change your aperture back to what it was before (smallest number), but this time, decrease your shutter speed to a much smaller number. In my example, I will set my shutter speed to 4 (quarter of a second) from 125. Take another picture. Now your image should be overexposed and some parts of the image should appear too bright. What happened this time? You let your lens pass through all the light it can gather without blocking it, then you let your sensor gather more light then it needs by decreasing the shutter speed. This is a very basic explanation of how aperture and shutter speed play together.

So, when does ISO come into play and what does it do? So far, we kept the ISO at the same number (200) and didn’t change it. Remember, ISO means sensor brightness. Lower numbers mean lower brightness, while higher numbers mean higher brightness. If you were to change your ISO from 200 to 400, you would be making the photo twice as bright. In the above example, at aperture of f/3.5, shutter speed of 1/125th of a second and ISO 200, if you were to increase the ISO to 400, you would need twice less time to properly expose the image. This means that you could set your shutter speed to 1/250th of a second and your image would still come out properly exposed. Try it – set your aperture to the same number you wrote down earlier, multiply your shutter speed by two and set it to that number, then change your ISO to 400. It should look the same as the first image you took earlier. If you were to increase the ISO to 800, you would need to again double your last shutter speed from 1/250 to 1/500.

As you can see, increasing ISO from 200 to 800 will allow you to shoot at higher shutter speeds and in this example increase it from 1/125th of a second to 1/500th of a second, which is plenty of speed to freeze motion. However, increasing ISO comes at a cost – the higher the ISO, the more noise or grain it will add to the picture.

Basically, this is how the Three Kings work together to create an exposure. I highly recommend practicing with your camera more to see the effects of changing aperture, shutter speed and ISO.