Metering, Drive, & Focusing Modes
Getting good exposures with your camera is a result of providing the right amount of light to your sensor. Of course, what qualifies as the “right” amount of light is going to be based on the assigned sensitivity of the sensor (ISO) to light and what you are trying to achieve with the picture. ISO, along with shutter speed, and aperture settings are the three components of exposure that must be in balance for a proper (or expected) exposure.
When you’ve decided on your ISO setting, the aperture and shutter speed are the two controls left that determine how much light is provided to the image sensor. Metering is how we measure light intensity so that we can know what aperture and shutter speed settings we need to use for the exposure we want to achieve.
When taking photos in automatic and semi-automatic shooting modes like Full Auto, Program, Aperture Priority and Shutter Priority, your camera will use the information from its internal meter to automatically adjust one or more settings to maintain a proper exposure for the scene.
Here is a list of common metering modes for DSLR cameras (note that your camera might use different nomenclature for these standard types of metering):
- Evaluative/Matrix Metering. This is considered a general purpose metering mode that uses intelligent programming to identify what type of scene you’re photographing, and which areas to prioritize for metering, so the camera can adjust exposure accordingly.
- Center-Weighted Metering. This can be visualized as a combination of Evaluative/Matrix and Partial metering modes. Here, the center area of the frame has priority but the lighting in the rest of the frame is also taken into account.
- Partial Metering. A small area in the center of the frame is metered rather than the overall scene. Useful when you want to expose properly for a subject in the middle of the frame and not have the meter take strong background lighting into account.
- Spot Metering. With spot metering, a very small area of the frame is metered. This is good for times when you want more
precise metering on a small area of the scene.
In practice, you’ll probably want to leave your camera set to Evaluative/Matrix as it will provide accurate results for most shooting situations. As you encounter scenes that are out of the ordinary in terms of subject placement and illumination, you might find that one of the other metering modes is more suited to a particular situation.
Despite what your camera meter might determine as the best aperture and shutter speed settings, you might find that you’d prefer the subject and/or background to appear a little brighter. Without changing the actual lighting in the scene (with reflectors, additional light sources, moving your subject, or adding flash), you’ll have to adjust the exposure overall. You can leave your camera on its current shooting and metering mode and simply override the camera’s exposure settings via the Exposure Compensation (EC) control. Doing this is as easy as dialing the appropriate control wheel to move the exposure level indicator up or down the meter scale. For example, if you’d like the subject to appear a bit lighter in the picture, you might dial-in +1 stop in the meter. To make the subject appear less bright, you might dial-in -1 stop.
Locking-in a Meter Reading
In some cases it might be useful to use the camera to meter a scene or subject and lock that information in before taking the actual picture. For example, you could take a meter reading of the scene, knowing that the background might become brighter as conditions change or you recompose the shot. By pressing your camera’s auto exposure lock (AE/AE-L), you will let the camera know to take a meter reading and store it for the next shot, or subsequent shots.
What About Metering For Flash Photography?
The light metering we’ve discussed in this chapter is for constant, (non-flash) lighting. These metering modes are suitable for metering and determining the exposure based on ambient lighting conditions. Flash metering is handled by another system in the camera (Through The Lens flash metering and control; TTL) that sends out a pre-flash, reads it through the lens, and determines what the flash output will be for the actual exposure. This all happens automatically as you press the shutter. Some new to flash photography don’t realize that the flash metering and exposure are separate from regular metering and exposure. Not only are the light readings taken separately, but compensation adjustments can be handled with different controls; EC for normal exposure compensation, and FEC for flash exposure compensation.
In contrast to the usual flash control provided by the TTL system as described above, manual flash photography is not metered at all by the camera. With manual flash, you set the power of the flash unit(s) yourself. Often, this is where a handheld flash meter can be useful, as there is no mechanism on your camera to meter manual flash. Many photographers don’t even bother with a flash meter and just do test shots with their digital cameras to zero-in on appropriate light power settings.
Pressing the shutter release button on your camera usually results in a single image being captured. But in certain drive (or release) modes, you can control a burst of exposures, often more than ten per second. The drive/release modes also let you set self-timer and remote release controls.
Most of the time, we take photos using a basic single-shot mode. This is great for portraiture and relatively static subjects where you’ll take at least a few seconds between shots to compose and capture your images. However, with sports or other action photography, it might be helpful to take a series of continuous shots. Sometimes a sequence of shots helps tell a story. Other times, the goal is just one good shot, and by capturing several images in quick succession, you’re hoping to find at least one in the sequence that you like.
Despite the fact that many cameras are capable of bursts exceeding 10 frames per second (fps), the upper limits of these burst modes aren’t always achieved however because they are somewhat dependant on other camera settings and modes including focusing, exposure, image quality settings, flash, and the capacity of your memory card.
Here are some of the drive/release modes offered on many digital cameras:
- One-shot: A single image is taken when you depress the shutter release button. You must lift your finger off the button and press it again to take another picture.
- Continuous/Burst: The camera continues to take pictures as long as you hold the shutter release button down. Some cameras offer a Low and a High setting allowing you to vary the number of frames per second. Some things to be aware of:
1. Your flash may not be able to keep up with a long burst of exposures. Damage to your flash can occur.
2. Focusing can be an issue; continuous auto-focus is usually the best way to go for moving subjects, but use a mode that maintains the original focus if necessary.
3. Since the shutter release button remains depressed during continuous shooting, the camera may not be able to adjust exposure as lighting conditions change.
- Timer: Self-Timer modes release the shutter and take a picture on a time delay. This allows the photographer to set the camera on a tripod or some steady surface and include himself in the picture. In self-timer mode, the photographer depresses the shutter release button but the camera waits a set number of seconds before actually taking the picture. Good for self-portraits and group shots including the photographer.
- Remote Release: This mode also frees the photographer from having to be at the camera position to take the picture. A cable or other remote device is attached to the camera and triggered from a distance. This also allows the photographer to be included in the picture, while still giving her full control over the exact time she wants to release the shutter. Many photographers will also use remote releases to prevent camera shake, photograph wildlife, or to work on stop motion animation projects.
Manual focus is available with many digital cameras and lenses but automatic focus (autofocus/AF) is more often employed because of its reliability and ease of use. Today’s cameras are equipped with multi-point AF technology that can achieve and maintain tight focus on still as well as moving subjects. In Full Auto mode, your camera will likely make all focusing decisions for you, including the focus mode and points of focus within the frame. Other modes will allow you to choose more options where focus is concerned. Check your camera’s documentation for instructions on how to select the various AF options with your camera. Here is a list of AF concepts you should be familiar with:
- Focus modes for still and moving subjects. Many cameras have a focus mode that is suitable for still subjects (often called “One Shot” or “Single Servo”) and another for moving subjects (often called “AI Servo” or “Continuous Servo”). While focus is activated, the one for moving subjects will continually track and focus on the subject as it moves. When the shutter button is fully depressed the camera will attempt to accurately predict focus for the moment of exposure.
- AF points. These are small areas of the frame designated as AF control points; objects in the scene appearing directly in line with these AF points are the ones that will be used for focusing decisions. These points can be chosen automatically by your camera, or manually selected by you. Depending on the model, your camera may have anywhere from a small number of AF points to more than 50. Sometimes AF points are grouped into zones which can result in more accurate automatic focussing in challenging shooting situations.
- AF Assist. Cameras and external flash units often come equipped with AF assist features which enable your camera to focus in darker environments when that might not otherwise be possible. AF assist can come in the form of a visible red or white beam of light from an external or built-in flash.
- Single point focusing. Many photographers like to use only one AF point to set focus on a subject. If the center AF point is used, the photographer will usually press the shutter button half way (or another designated focus button), achieve focus, and recompose just prior to taking the photo. This method of focusing is not recommended for moving subjects because the other AF points are not used and therefore cannot provide multi-point tracking.
Check your manual to review any advanced auto/intelligent focusing features of your particular camera.