In this article I’ll be covering the different exposure controls for video. You might be wondering why we have aperture, ISO, the shutter and ND filters, when they all do similar things. These exposure controls have different secondary affects on the picture, so these need to be taken into consideration. They can change the depth of field, motion blur, sensor noise and even infrared pollution. You might want to prioritise certain camera settings to increase or reduce these effects, to really take control of the look of your film.
Exposure Controls Overview
Let’s get a quick overview of the different exposure tools we’ll be looking at. Photographers use the exposure triangle as a guide for balancing settings, so we’ll start there.
The exposure triangle gives us:
But there are other video camera settings that effect picture brightness:
- Frame rate
- ND filters
Lighting is the odd one out, because it’s not a camera tool. But as we’ll see, it’s very important when you’re shooting a film. Let’s look at these exposure controls in more detail.
Exposure Controls 01 – Aperture
Out of the different exposure controls, aperture is the one we immediately think of. Sometimes called the iris, it’s the hole in the lens that lets in light. You open and close the aperture to control how much light reaches the sensor. The aperture is either controlled by a ring on the lens or by a wheel on the camera body. The size of the aperture is measured in stops. The lower the stop number, the more open the aperture, which gives you a brighter picture. Higher stop numbers mean a more closed aperture, giving you a darker image.
Depth of Field
As well as controlling exposure, the aperture has a major effect on DOF (depth of field). This is how much of the scene in front and behind the subject is in focus. With a high stop number, you’ll see a deep depth of field. So more of the scene is in focus. And with a low stop number, you’ll see a shallow depth of field, so less of the scene is sharp. Objects beyond the focus range will appear soft.
Exposure Controls 02 – Shutter
The shutter sounds like something mechanical and indeed it was before digital cameras came along. It controls how long the sensor is exposed to light. You can change the shutter settings with either a physical control, or the camera’s touch screen.
It’s either measured as the shutter speed, which is a fraction of a second. Or it’s measured as the shutter angle, which is in degrees. A faster shutter speed gives you a darker picture, while a slower shutter speed gives you a brighter picture. A smaller shutter angle produces a darker image, while a larger shutter angle gives you a brighter image.
The other effect the shutter has on the picture is motion blur. This affects how much blur you see on moving objects. The default shutter speed is twice the frame rate. So, when shooting 24fps, that will be a 48th of a second. The default shutter angle is 180º. At these settings, moving objects have the same amount of motion blur as human vision. With a faster shutter speed or smaller shutter angle, there is less motion blur. But with a slower shutter speed or a larger shutter angle, there is more motion blur.
Exposure Controls 03 – ISO
The ISO is an exposure control that artificially makes the image brighter or darker. The ISO is adjusted with a physical wheel or with the touch screen. It’s measured in ISO units, which are in the hundreds or thousands. The higher the number the brighter the picture.
As you raise the ISO, you’ll start to see more sensor noise in the picture. This is because boosting the brightness lifts the sensor noise floor, making it more visible.
Exposure Controls 04 – Frame Rate
When you’re shooting, the camera records a certain number of frames per second. This is the frame rate. Frame rate isn’t used as an exposure control, but changing it does effect brightness levels. It’s usually set from a control on the touch screen or in the cameras menu.
The main reason for adjusting the frame rate is to create speed effects. Shooting at a faster frame rate, then playing back at normal speed, gives you a slow motion picture. Increasing the frame rate makes the image darker. Decreasing it makes the picture brighter.
Exposure Controls 05 – ND Filters
Neutral density or ND filters are like sunglasses for the camera. The control exposure by cutting the amount of light reaching the sensor. Some cameras have adjustable ND filters built in, while others need external filters.
Internal ND filters are adjusted with a dial on the camera body. External NDs are make of glass or resin. Using a matte box, you clip the ND into a filter stage and slide it into position in front of the lens. The internal ND on a camera is measured in fractions. The density of an external filter is measured in decimal numbers.
In theory ND filters only lower the brightness of the image. But in practice they can have other side effects. If the colour of the filter isn’t perfectly neutral, it can introduce a colour cast. Another issue is IR (infrared) pollution. The ND filter cuts visible light, but can allow infrared radiation through. This becomes visible when it pollutes the red channel, making darker areas of the image more red.
There’s a very easy fix, attach an IR filter to the lens. This screw-on filter cuts IR radiation, removing the red cast from the picture. This Hoya filter cuts ultraviolet and infrared radiation.
Exposure Controls 06 – Lighting
The final exposure control is lighting. Lighting is the art of shaping artificial light or sunlight to illuminate and scene and create the mood. If you’re using a light meter, the brightness of lights in a scene is measured in lux or candelas. When you’re using a waveform monitor on your camera, it’s in percentages or nits.
If you want to know more about using a waveform monitor or false colour to set exposure, watch this video: Don’t ETTR, Use a Grey Card
Changing light levels can result in some secondary effects. For example, if you lower the dimmer on a tungsten light, it makes the colour more orange. On some lights, adjusting the dimmer can cause them to flicker. In that case, you will need to adjust the shutter to compensate.
Some lights flicker when you dim them. Use the RED flicker free calculator to choose shutter settings that eliminate flicker.
Before we look at prioritising exposure tools, let’s give the camera some standard settings.
- Set the frame rate to 24 frames per second, which is standard for a movie.
- If your camera uses shutter angle, set it to 180º. If it uses shutter speed, set it to a 48th of a second.
- Set the camera to the recommended native ISO. This is often 800, but check the manual. If your camera has dual native ISO, pick the lower value.
- Adjust the aperture and if necessary ND filters to get a good exposure
Now we’re got the camera setup, let’s look at why we might prioritise one exposure setting over the others.
Let’s imagine you want some smooth slow motion. The best way to do this is by increasing the frame rate. So, the frame rate becomes the priority. If you want the shot to playback at half the speed, you need to double the frame rate. So we need to increase 24 fps (frames per second) to 48 fps.
This darkens the image, but that’s not the end of it. Depending on how you want the slow motion to look, you’ll probably adjust the shutter speed or shutter angle. This could either increase the brightness or reduce it even more. To compensate for reduced brightness, you’ll need to use the other exposure settings or add more light.
If you’re shooting fast action, or you’re shooting against a green screen, you might want to reduce the amount of motion blur. You can do this by increasing the shutter speed or reducing the shutter angle. Having less motion blur can look unnatural, because we naturally see motion blur with fast moving objects. Doing this will also darken the picture, so once again you’ll need to use other exposure settings or lighting to increase the brightness.
If you want more motion blur in the picture, you can reduce the shutter speed or increase the shutter angle. It can also be created with effects in your editing software.
Motion Blur and Shooting Green Screen
We shoot characters against a green screen so we can composite them with a different background. For this to be successful, we need the edge around the character to be well defined. If they’re moving around, motion blur can make the edge fuzzy.
To reduce motion blur and improve the edge, shoot with a faster shutter speed or lower shutter angle. Make sure you have enough light on the green screen stage to compensate for the reduction in exposure.
When shooting green screen you might want to lower ISO to reduce sensor noise, which will further darken the picture.
Once the character’s composited into the visual effects shot, a motion blur effect can be added to make the shot look more natural and blend it with the other shots in the movie.
Depth of Field
Let’s imagine controlling depth of field is really important to the scene. In that case, the aperture is your priority. As a storytelling tool, depth of field is used to show or hide what’s happening in the background. Or it can be used to show a character’s relationship with the world around them. If a character is content and at one with their world, then a deep depth of field will suggest this. You create a deeper depth of field by closing down the aperture. This makes the picture darker, so you’ll need to compensate by increasing the brightness with a different exposure control. If however a character feels out of place or disconnected from the world, then a shallow depth of field will give us that sense of isolation by making their surroundings out of focus.
We can also use shallow depth of field to direct the audience’s eye, showing which character is most important.
Do you want your next project to have a sharp, modern look? If so you’ll want to make both ISO and the aperture your priorities. When your camera is at its native ISO, there might be a small amount of noise in the image. To make the picture even more crisp, you can reduce the ISO by half. So if the native ISO is 800, lower it to 400.
All lenses have a sweet spot, where they are at their sharpest. You can find that sweet spot by adjusting the aperture:
- Set the lens to the maximum aperture. This is the lowest stop number, which gives you a bright picture
- Close it down by two or three stops
- Make sure the focus is as sharp as possible
The sweet spot is not the same for all lenses. Shoot some tests at two, three and four stops below the maximum aperture. View them on a large screen to see which has the sharpest focus.
Because the ISO and aperture are tied to the look of the picture, you’ll need to use other methods to control the brightness. You can use ND filters if the picture is too bright, or look for ways to bring in more light if it’s too dark.
Just a note about sensor noise: if you want to add noise to your film, don’t do it with ISO. Use effects in your editing or visual effects software, as this will give you more control over the finished look.
The Importance of Light
Hopefully you’re beginning to see how important lighting and ND filters are for controlling exposure. On most fiction projects you probably won’t touch the frame rate or shutter settings. You might also decide to set the ISO to its native settings and leave it there. If the aperture is tied up with controlling depth of field, then all you’re left with is ND filters and lighting. It doesn’t mean you can’t adjust the aperture, but you might limit yourself to a range of two or three stops.
- Frame rate
- ND filters
If you’re shooting exteriors, you can use lighting modifiers such as reflectors, diffusers and nets to control daylight. If you’re shooting interiors you can use artificial lights or reflect sunlight into the room.
I hope you enjoyed this look at the different exposure settings and why you would prioritise certain settings. Have a go yourself and see how you get on.
If you’ve got any questions or suggestions please comment.
If you found this useful, check out the rest of my fantastic Camera Training videos:
- Intro to Cinematography
- Don’t ETTR, Use a 50% Grey Card
- Balancing Exposure for Video, Thinking in Stops
- What is ISO?
- White Balance Video like a Pro
- Camera Resolution
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