The exposure triangle explained

While modern cameras do a good job of guessing proper exposure, understanding how exposure works and how to set it manually is a critical skill in videography.

In a nutshell

  • You can control your camera’s exposure by balancing its shutter speed, aperture and ISO
  • All three corners of the exposure triangle change their artistic qualities and exposure
  • If balancing the exposure is difficult, add more light

The exposure triangle is the most fundamental concept of image capture, yet many people know little about it. That’s because the electronics of our cameras are fairly good at guessing what things should look like. Most of the time, you can trust your camera. However, if you’re at all serious about photography or video, you need to understand how both work and the tradeoffs that come with each. Exposure is an important part of cinematography.

Simply put, exposure is the amount of light your camera’s digital sensor absorbs. Sometimes you want this to be a lot of light, and sometimes, very little. For instance, lots of light a beach scene at noon or not much in a spooky parking garage at midnight. Get the exposure wrong, and your beach scene at noon will look like you’re seeing it through a welders mask; Your parking garage won’t be spooky at all.

Simply put, exposure is the amount of light that your camera’s digital sensor absorbs.

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There are three things that determine your exposure: your camera’s shutter speed, its aperture and sensor’s sensitivity. Sensitivity is measured in either ISO, ASA or Gain, depending on the camera. Each of these variables has a gremlin riding on its back. That is, it changes something other than your exposure. Consequently, many people throw their hands in the air and set their camera on auto to be done with it.

The three points of the exposure triangle

There are several ways to arrive at the correct exposure. Unfortunately, changing one setting on the exposure triangle means that at least one of the other two may need to be changed as well. All the while keeping in mind the side-effects that go along with each of these variables.

What’s more, when some numbers get bigger, things get darker but when other numbers get bigger, things get brighter. Here’s a quick look at what’s going on:

Shutter speed: This is the fraction of a second the sensor is exposed to light. As numbers gets BIGGER the exposure gets DARKER.

Shutter speed – 1/640
Shutter speed – 1/5

What it does

Determines the amount of time the sensor will be paying attention to light hitting it – video cameras don’t have shutters in the traditional sense, but achieve the same effect by ignoring and then paying attention to light hitting them for a period of time measured in fractions of a second – from 1/30th of a second at the low end all the way to 1/4000th of second or even less at the high end.

Side effect

Also controls how much blur moving objects in a frame have. A slow shutter speed (the shutter is open for a longer time) will cause quickly moving objects — like a sprinter or a football player — to be blurry because they moved during the exposure. A high shutter speed (the shutter is open for a short period of time) will freeze moving action. However, very high shutter speeds can cause very quickly moving objects to appear jittery because your video is playing back at constant rate of 30 frames per second.


Aperture: This is the width of the diaphragm letting light in through the lens to hit the sensor. As numbers get BIGGER the exposure gets DARKER because the aperture actually gets SMALLER.

Aperture – F/5.6
Aperture – f/22

What it does

Controls the amount of light that comes through the lens.

Side effect

Also determines the depth of field, which is how narrow an area will be in focus. Larger apertures (confusingly given smaller numbers like 1.4 or 2.8) let in more light but also cut down on the range at which objects are in focus. Smaller apertures (confusingly given larger numbers like 16 or 22) let in less light but give a greater range of areas that are in focus.


ISO/ASA/Gain: This is the sensitivity of the sensor to light. As numbers get BIGGER the exposure gets BRIGHTER.

ISO – 100
ISO – 6400

What it does

Controls how much light the camera will use to make an image.

Side effect

Higher ISOs will add “noise” to the video — seen as tiny dots — in film cameras this is called “grain” because at high ISOs you can actually begin to see clumps of the crystals that make up the image. Digital noise on video sensors actually comes from the camera making guesses about data that it doesn’t have enough information to represent exactly.

Visible “noise” at high ISO

Don’t confuse shutter speed with frame rate

When shooting video, your camera is recording a certain number of frames per second. This is usually 24, 25, 30, 50 or 60 frames per second. (Watch our video on frame rates for more about what each means.)

Each of these frames is a still image, and shown one after the other, they look like they’re moving. It’s a common mistake to confuse how many frames are being shown each second with the shutter speed. Shutter speed is simply how long the aperture is open whenever it captures a frame. Shooting at 1/125th of a second doesn’t mean you’re shooting 125 frames per second. You’re still shooting 30 frames per second, but each frame represents 1/125th of a second of exposure to light.

An honorary member of the exposure triangle: Neutral density filters

THis one isn’t on the exposure triangle, but it’s still important. If you’re shooting stills and you want a wide f-stop like f/1.8, you can pick a high shutter speed to make sure your exposure is correct. But if your subject is moving, a high shutter speed might cause your video to jitter; So what can you do? You can either lower your ISO or lower your lighting. However, your ISO can only go so low. Plus, if you happen to be using natural light, you can’t very well dim the sun can you?

Well, you sort of can. Neutral Density filters are basically sunglasses for your camera. They make everything a little darker by making it more difficult for light to get through. By adding ND filters, you can open your f-stop to get shallower depth of field without increasing your shutter speed.

Quick exposure tips

For the most natural motion blur, keep your shutter speed at double the frame rate. Intuitively, for 30 fps, this this means 1/60th of a second. But if if you’re shooting at 24 fps its 1/50th, since 1/48th typically isn’t an option.

Cameras have a “native” ISO that they’re optimized for — often 160. Keeping it as close as possible to this setting will give you the highest image quality. To see how far you can deviate from this setting, do some real world tests and examine the footage critically. Larger sensors can usually shoot noise-free at higher ISOs than smaller ones. Some of these cameras are fairly good even up to and beyond 1600.

Focusing can be extremely tricky with very shallow depth of field. Consider using a viewfinder magnifier or external monitor when focus is critical. Also, plan your shots carefully so that you can smoothly follow people and keep them in focus as they move. In Hollywood, there are people called “focus pullers”. Their job is just to make sure the camera is focused properly as the camera and actors move.

Pull a rabbit out of your hat

As Nigel Tuffnel asks in “This is Spinal Tap” — When you’re on “10” on your guitar, and you’re on “10” on your amp, where can you go? When your ISO maxed out, your aperture is as wide as it can be and your shutter speed is as low as it goes, where can you go? The answer is simple. Instead of trying to capture more light that isn’t there, you can add more light. Do this by either by using artificial lights or moving the action closer to better sources of natural light.

Conclusion

Learning how the exposure triangle works is one of the most important things to understand about video production. Luckily, there are only three numbers involved. Your mission, should you choose to accept it, is to practice setting your exposure manually using all three of the variables here. First, set your ISO low and come up with a proper exposure by changing the aperture and shutter speed. Then set your ISO high and again use the aperture and shutter speed to find the proper exposure. Finally, try keeping your shutter speed the same and arrive at the proper exposure using the aperture and the ISO. Remember, if things are too bright, you can add neutral density filters. And if they’re too dark, you can add more light.

1 COMMENT

  1. I am using a SonyHVR-Z5 camera and in my operating guide I do not see anything that says what the ISO is or if that is a changable settings. I know that ISO is a common changeable setting on DSLR cameras but it not so, I true HD digital camcorders which most of us still have. Please clear up this misunderstanding. Thanks.
    Larry
    Acclaimed Video

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