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Exposure

Back when indie films were shot on 35mm film, the hardest part of shooting was getting proper exposure. It was entirely possible that your film stock would come back from the lab with no image at all on it. With video, the only way that will happen is if you leave the lens cap on. But getting good exposure is still important.

So what, exactly, is exposure? At the most basic level, exposure is the amount of light that is allowed onto the image sensor(s). When too much light falls on the image, it is overexposed and when there’s not enough light, the image is considered underexposed.

Underexposure usually results in a dark or muddy-looking image, whereas overexposure causes bright areas to turn white, or get “blown out.” Properly exposed film or video footage usually contains some areas of overexposure—usually reflective highlights and bright whites. With film, overexposed images are easier to fix than underexposed images because the brightness can be pulled down in the film lab. However, with digital video, overexposure can cause many problems because blown-out white areas contain no visual information. So, for digital video, underexposure is preferable to overexposure. Of course, proper exposure is always best of all.

Film shooters use light meters to judge exposure, but with digital video it’s possible to judge exposure by eye. Instead of a traditional light meter, better-quality digital video cameras have the zebra stripes feature to let you know what parts of the frame are overexposed. Some video cameras, and most DSLRs, also have a histogram view that shows the range of lights and darks in any given frame (see Figure 7.9).

Figure 7.9

A frame of HD video with the zebra stripes feature enabled (right) and a histogram of that same frame ((left). In this image, the sky is overexposed.

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Most higher-quality digital video cameras have video outputs that allow you to send the video signal out to a field monitor to immediately see if the image is properly exposed. Field monitors are bigger and make it much easier to check the quality of the image than the LCD displays on the camera itself. We recommend always using a field monitor when shooting video—think of it as your first and only line of defense against poor image quality. With a field monitor, you can make an immediate decision as to whether or not an overexposed image is acceptable for your project.

Most video cameras offer an auto-exposure setting, but auto exposure can be adversely affected by a change in the content of your scene. If an actor walks into a frame and blocks some light, for example, you might see your camera automatically open the iris up, resulting in a disturbing, overall contrast change. For this reason alone, you’re usually better off controlling exposure manually. You might decide, for example, to purposely overexpose the sky, in favor of keeping a good exposure on an actor’s face. Be aware that the auto-exposure mechanism in your camera will make exactly the opposite decision, and will properly expose the sky, causing the actor’s face to fall into shadow.

Controlling the exposure manually involves setting the aperture (or f-stop), the shutter speed, and the gain (or ISO). These three controls have a reciprocal relationship—if you change one, you’ll need to make a corresponding change with one of the others to maintain the same overall illumination.

What that means, practically speaking, is that you have three ways to adjust the camera to get proper exposure.

Aperture

Aperture refers to the opening at the rear of the lens. The camera’s lens focuses light through the aperture and onto the image sensor mounted on the focal plane. The size of the aperture is controlled by the iris, a series of interlocking metal leaves that can expand and contract like the iris in your eye. The size of the opening of a camera’s iris is measured in f-stops. Highernumbered f-stop values stop more light. That is, a higher-numbered value represents a smaller aperture, which provides more light stoppage, resulting in less light passing through the lens (see Figure 7.10).

Figure 7.10

The iris on the left is stopping more light than the iris on the right is. Therefore, it has a

higher f-stop value, and makes a smaller aperture.

154 The Digital Filmmaking Handbook, 4E

Most high-end video cameras show f-stop markings on their lenses, which make it easy to set a particular aperture. Mid-range cameras do not have f-stop markings on their lenses, but provide control of the lens aperture with special dials, and with f-stop settings that appear in the digital menu display. Lower-end cameras do not display f-stops, but allow for manual iris adjustment and iris locking. Bottom-line cameras do not allow for any manual control of the lens aperture and rely solely on automated iris settings.

All video cameras include an auto-iris function, which constantly adjusts the size of the camera’s iris according to the current lighting conditions. While the auto-iris function might work fine in “normal” lighting situations, it’s usually the wrong choice in more complicated scenes. For example, if you have a subject who is in the shade, but a background that is sunny, the auto iris will probably expose for the bright background, since it fills the majority of the frame (see the top image in Figure 7.11). Because the camera is exposing for the brighter part of the image, the shadowy person in the foreground ends up underexposed and difficult to see. In order to expose for the darkened foreground figure (see the lower image in Figure 7.11), you’ll need to use the manual iris control to overexpose. This will blow out the background, while keeping the subject exposed correctly.

As your camera moves, or the subjects in your scene move, your camera might re-evaluate what it thinks is a proper exposure. For example, as people walk in front of light sources in your scene, the iris will fluctuate, causing your scene to get lighter and darker. Though what the camera is doing is technically “correct,” it will look awful. If your camera allows it, you should usually work with the auto-iris function turned off and control the iris manually while shooting.

Sometimes, it’s not that easy to decide what the best aperture setting is for a tricky shot like the guy in front of the window. When that’s the case, professionals use a technique called bracketing. They shoot the shot three times—once with the exposure they think is best, then one stop down from that and one stop up from that. The result is a higher likelihood of getting good exposure in one of the shots. This technique can be a bit time-consuming so it’s not recommended for every shot in your film, just the ones that are really important and challenging.

Shutter Speed

The shutter determines how long the iris is open when you are shooting. With motion video and film, the lowest shutter speed is determined by the frame rate you are shooting at. For 30fps video or film, the slowest shutter speed possible is 1/30th of a second. But usually, the slowest speed available on most consumer cameras is 1/60th of a second, which corresponds with the fastest shutter speed available on most film cameras.

There are advantages to both slow and fast shutter speeds. Faster shutter speeds are better at capturing fast motion without blurring. Sports shooters typically shoot at very high shutter speeds. Fast shutter speeds also limit the amount of light allowed into the image sensor and are one way of reducing the exposure or stopping down.

Slow shutter speeds are considered more film-like. Additionally, they provide a way to increase the exposure in low-light situations.

In Chapter 4, we showed you an example of the difference between a fast shutter speed and a slower shutter speed. The resulting motion blur of the slow shutter speed will help make your video look more like film. (Refer to your camera manual for instructions on manually adjusting the shutter speed.) Sometimes adjusting the shutter speed can reduce on-set issues such as flickering monitors or lightbulbs.

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Figure 7.11

In the upper image, the camera’s automatic exposure exposed for the sunlit background, leaving our subject in shadow. In the lower image, we used the manual exposure control to expose for our subject. Although the background is blown out, at least now we can see our subject’s face.