156 The Digital Filmmaking Handbook, 4E

A Shutter by Any Other Name

Most digital video cameras don’t have a mechanical shutter that opens and closes. Instead, they use an “electronic” shutter; that is, the image sensor inside the camera simply turns itself on and off for the appropriate duration.

Gain

All newer video cameras come with a gain boost feature for low-light situations. Gain boost makes everything in your image brighter, including the black areas of your image. A good video image should aim for dark, solid blacks and bright, clear whites. If the gain is boosted, the blacks tend toward gray. DSLR cameras allow for a choice of ISO settings, which is effectively the same as gain boosting.

With many digital video cameras, the color cast of the image tends to go red when you shoot with the gain boosted. In addition, video noise is introduced. Noise is the video equivalent of film grain, except it is much more noticeable and distracting (see Color Plates 5 and 6). If you are planning to go to film later or project your video, you should avoid any gain boosting. It only looks worse when it’s bigger. Instead, use proper lighting to get a good exposure on your videotape and turn off the gain boost feature.

Gain boosting capabilities have improved greatly in newer cameras, but too much gain boosting will always look bad. However, sometimes using the gain is the choice between getting exposure and not (see Figure 7.12).

Figure 7.12

Thanks to a little gain boost, it was possible to shoot usable footage in this completely dark theater. However, gain boost is no substitute for good lighting. Dark is dark, and boosting the gain can only recover so much image. In addition, as can be seen here, boosting the gain dramatically increases the amount of noise.

Which One to Adjust?

As we mentioned earlier, you can adjust the aperture, the shutter speed, and the gain/ISO to change the exposure. So how do you choose which one to adjust? Because boosting the gain can introduce noise, this is typically used as a last resort. Ideally, you’ve lit your scene so that you don’t need to increase the gain. Because different shutter speeds create different “looks,” typically that is a choice you make in advance. So it’s best to disable the gain, set the predetermined shutter speed, and then use the aperture settings to adjust for good exposure. However, if you find yourself shooting in a low-light situation, you have three ways to improve the exposure of your image.

Exposure and Depth of Field

Having control of the exposure gives you the freedom to control the depth of field in the image, which allows you to separate the subject from the background. Depth of field refers to how “deep” the range of focus is in an image. Modern HD cameras are very sensitive to light and very good at shooting in low-light situations. That means it is easy for the entire image to be in perfect focus. That might sound great at first, but when you look at a large, busy image like a crowd of people on a 50-inch plasma display, if the whole image is in perfect focus, it will be hard to decide what to look at. As a result, many filmmakers prefer a shallow depth of field so that they can put the main subject of the shot in focus and keep the rest of the image soft.

In Figure 7.13 (left), the depth of field is shallow, and only the subject is in focus. It is easy to focus your attention on the actor because he is completely separated from the background. In Figure 7.13 (right), the depth of field is longer, and more detail shows in the background.

Both of these images were shot under the same lighting conditions with the same lens. In Figure 7.13 (left), the depth of field is shallow because the lens aperture is wide open. In Figure 7.13 (right), the lens aperture is closed down all the way, resulting in a less shallow depth of field. Moreover, depth of field is shallower at longer focal lengths. Therefore, if you want a really shallow depth of field, you need to position your camera farther from your subject, zoom in, and open the iris as far as possible.

Figure 7.13

By controlling the depth of field in your image, you can selectively blur out parts of your image.