Getting your shots is probably the most important and one of the more complicated thins that is part of shooting for theatre. Probably the most important elements that go into getting the shot are focus and metering. There are many techniques and tools that can be used to get good focus and exposure.
Modern cameras all sport autofocus capabilities when used with compatible autofocus lenses. Most camera’s autofocus systems rely on having contrast within the selected focus area to dial in the focus. If your subject is in extreme shadow or is otherwise a low contrast subject the autofocus system may have a hard time finding and locking in the focus.
Most modern cameras have multiple selectable focus areas. These areas are usually superimposed on the focusing screen in the viewfinder and are highlighted when selected. There is typically a rocker switch or other control device that is accessible by your right thumb that is used to select focus areas. Usually I try to set my focus areas on an actor’s face, and if possible their eyes.
Many cameras also have different autofocus modes. The basic modes are typically manual, single servo auto, and continuous auto. Manual focus disengages the focusing drive and allows you to focus manually. Single servo focus focuses n the subject and then locks, no refocusing happens unless you release the shutter button or AF-ON button and then depress it again. Continuous focus mode keeps the focus drive active so if the subject moves or if something gets between you and the subject, the camera will continue to adapt and focus on whatever is in the focus area.
For most theatre applications I like to use the single servo focus mode. In a regular photo call situation this is probably the most efficient way to get a sharp image. If I am shooting during a run or if I am trying to capture a scene with a lot of movement (like dance) I will use continuous focus so that as the performers move they stay in focus.
Your camera has a built in light meter. The light meter is a sensor coupled with a processor that assesses the light that comes into the camera through the lens (TTL) and determines an appropriate exposure setting (shutter speed and F-stop). Onboard light meters typically have three modes of operation: matrix or pattern mode, center weighted, and spot metering.
Matrix metering uses complex algorithms to measure the light over the entire scene to pick appropriate. To be completely honest I have no idea how matrix metering works aside from the fact that it generates the “best” average reading for an entire scene.
Center weighted metering looks at a given percentage of the center of the frame and derives exposure values from there. On some cameras you can actually set the percentage of the frame that is used by the center weighted metering. This metering mode is useful when your subject is mostly in the center of the frame.
Spot metering uses a small spot in the frame to get an exposure reading from. Usually the spot used for metering is the same spot that you use for your autofocus point. So on cameras with more focus points you have more points that you can use for spot metering.
Most cameras have at least four exposure modes, Program or Preset (P), Shutter Priority (S), Aperture Priority (A), and Manual (M). Each mode effects how the camera picks (or doesn’t) the appropriate exposure settings for an image. Some cameras, especially entry level cameras have “scene” modes that adjust settings for things like portraits, landscapes, night scenes, etc.
In Program mode the camera chooses both an aperture and shutter speed to create a correctly exposed image. This mode is essentially an automatic mode which requires less thought on the part of the operator. Often times you can adjust the exposure settings that the camera picks by rotating the command dial. This way, if you want a wider f-stop or faster shutter speed, you can adjust while keeping the values equivalent.
In aperture priority mode you can pick the aperture that you want to shoot at and the camera will pick a corresponding shutter speed. Similarly, in shutter priority you pick the shutter speed and the camera chooses an aperture. These settings can allow you to pick an ideal setting for your application and then allow the camera to choose the rest of the information. So if you know you need a fast shutter speed to stop some action, you can set that and the camera will pick appropriate apertures. If you know you want to create specific DOF effects you might work in aperture priority.
Manual mode gives you full control over the exposure settings. The camera usually has some form of meter in the viewfinder that will tell you when your settings will yield what the camera considers to be a proper exposure. You can also set any settings you want, like if you are using a hand held light meter or if you purposefully want to over- or under-expose the image.