Now, it’s great to have the camera be able to determine exposure for you, but this isn’t always the best plan. The camera, smart as it may be, still isn’t as smart as you, and for the most part it doesn’t know what specifically it’s photographing. It also doesn’t know the artistic intent that you as a photographer want to use to represent your scene, and exposure has a lot to do with the aesthetic of the shot. What your camera tries to do is put the scene exposure in to a happy medium where everything should look about right in most circumstances. If you look at the bottom of your viewfinder you’ll see a little linear scale. That’s there so, even when shooting manually, you have a way to see what the camera considers to be a balanced exposure. This happy medium is based on a value called “middle gray” and it works absolutely perfectly every time you take a picture of a medium gray wall and want it exposed as a medium gray value. But what if you’re shooting a black can on a white field of snow? There’s nothing gray in that photo. The camera will see the bright field of snow and try to set that as the “middle gray” value so the overall scene looks like a good average exposure. But snow isn’t gray; you want it to be white! There are several ways to fix this: metering modes and exposure compensation.
These go by different names per your camera brand, but they’re all quite similar. These modes will let you define what area of your frame you want the camera to use for determining its exposure. Do you want the entire scene to be considered? Do you want just the small area of the frame where the black cat is sitting to be used? Metering from a “spot” in the snow will make the snow gray and metering from a “spot” where the cat is will make the snow “blow out” beyond white.
Exposure compensation is a bit more manual approach to compensating for the camera’s exposure, and it works best when you understand how the camera’s meter works, which you now do. In our example of the black cat on white snow, we know the snow is going to dominate the scene and if we’re using a metering mode that considers the entire frame to determine our exposure then the snow will come out at that middle gray value. Exposure compensation is simply a way to tell the camera to adjust its determined exposure by a value you specify. In our case we may want to add a couple stops compensation to bring that gray snow back up to white.
Many classic cinematographers and some photographers critique the meter-obsessed newcomers who religiously park their exposure safely in the middle of their scopes. There are often times when a dark dramatic frame will be visually appealing, but not look like a usable exposure on a histogram or waveform. Most of these come from the historical practice of determining final exposure on the camera however. In the digital world, post manipulation of the image allows you to “shoot safe” and create the dramatic exposure in the comfort of a color suite.
Again, the camera generally has a meter built in to guide you with setting exposure. This meter is a reflective meter that measures the light coming through the lens. Incident meters can be used at the location of the subject to measure the amount of light hitting any point in space. The in-camera meter makes its decisions based on what portion of the frame you’ve assigned it to use to evaluate exposure. “Spot metering” will only use a small portion of the frame to determine exposure while a “matrix metering” will consider the majority of the image and will better average the whole. The meter generally tries to put your image right in the center of the exposure range, the middle of the histogram, or 18% gray.
Incident metering can be helpful in determining lighting ratios or the difference between two lights. A common example would be determining how strong your key light is relative to your fill light. Another massively useful application for traditional meters occurs on large sets. The director of photography will order lights specific to a production and set an ambient light level and all other lights relative to that level manually. A meter can be indispensable in this scenario.
Digital cameras shooting log formats reduce the midtone exposure in order to make room for more highlights and increased dynamic range. When using these recording formats, do not try exposing an 18% gray card at 50%.
The principle of aperture applies the same to photo and video. Shutter speed is a bit simpler in the video world. Since we usually shoot at 24 frames per second for a “cinematic look”, and the shutter opens and closes twice-per exposure, all you have to do is double your framerate to get the denominator (bottom number) of the appropriate shutter speed. So a 1/48 shutter works well for 24 frames per second. If you’re shooting higher frame rates then the same rule applies–a 96 frames-per-second shot benefits from a shutter-speed of at least 1/192.
When the shutter speed is increased beyond this value the individual frames will appear too sharp and the motion will look jittery. Just make sure it’s a conscious artistic decision if you do this.