How to Use a DSLR – Part 2: Exposure Compensation, Metering, Autofocus and Shutter Modes
In this second part of the How to Use a DSLR series, we will cover four more buttons found on your camera. They are the exposure compensation, metering modes, autofocus and shutter mode. Some of these settings can be a little difficult to understand at first, but understanding what they are used for will help you in taking better photographs.
Every modern camera comes with an built-in light meter that measures the amount of light that enters the camera and feed the information to the camera. The amount of light measured is used to calculate the appropriate exposure level.
As this calculation is performed by the processor in the camera, it will result in an exposure level that the processor assumes right for the situation. However, there are situations when the environment and lighting sources are tricky and the camera exposes differently from what we want. For example, in scenes where the majority of the frame is very bright or very dark, the camera will compensate for the excess brightness or darkness and give an averaged exposure, resulting in a less than ideal image.
The exposure compensation (EV) now gives us an option to correct this error by manually increasing or decreasing the exposure. The +/- signs on the EV button allows us to do this in increments of 1/3 of a stop. A +1 EV will tell the camera to adjust its shutter speed and aperture to increase the exposure by 1 stop, depending on your camera mode. In aperture priority, the camera will keep the desired aperture and adjust the exposure by changing its shutter speed. Similarly, in shutter speed priority, it will open or close the aperture to achieve the desired compensation while keeping the same shutter speed.
The illustrations above show the changes to the exposure when exposure compensation is used. I used aperture priority mode to keep the aperture at f/2.5 on a 50mm lens. Note how the camera then change the shutter speed accordingly for each EV value, resulting in a brighter and dimmer exposures in each case.
As explained above, the function of a light meter is to measure the amount of light entering a camera. Having this information, the camera still needs to know which part of the frame needs to be exposed correctly, whether it’s the entire frame or certain parts of it.
We the users can tell the camera where we wish to emphasize by using the metering mode button. There are three common metering methods found in most DSLRs: evaluative or matrix metering, center-weighted average metring and spot or partial metering. The actual names used in the camera might vary from one manufacturer to another, but they would fall into these three categories.
Average metering is the most commonly used mode as it takes in all the information from the entire scene in metering the exposure. As it does not give any weightage to any particular part of the scene, it can be used often when you are not sure about which metering to use.
Center-weighted average metering
Center-weighted average metering places more importance to the center portion of the scene in assessing the exposure. The peripheral area of the scene is still taken into consideration but given less weightage. This mode is useful when the subjects are located towards the center of the scene, as often is the case. As such, the subjects will usually be exposed correctly while the background surrounding the subjects would not affect the exposure much.
Spot metering is on the other end of the metering spectrum, giving much weight to a very small area in the scene. Usually this area is at the very centre of the scene or at the focusing point if you have chosen an off-centre focusing point. The small area gives us the ability to meter for precise points in the scene, making it very useful in high-contrast situations such as those with strong backlight or a bright object in a dark background like the moon. In this illustration, the camera meters at the center spot which is relatively brighter. For this spot to be metered correctly, the exposure is reduced, leaving the rest of the scene a little darker.
Most modern cameras come with both manual and auto focus, but within auto focus alone, there are multiple autofocus modes available.
Single-servo (AF-S or Single)
In AF-S mode, the camera only focuses when the shutter release button is pressed halfway down. It then locks the focus while the button remains pressed, allowing you to recompose the scene and keep the focus on the object. This mode should be used when objects are stationary. The drawback of this mode is that the camera must find a focus point before you can fully press the shutter release to capture the scene. Nevertheless, I personally keep my camera on this mode as it is very versatile and allows me to focus wherever I want.
Continuous-servo (AF-C or AF-Servo)
AF-C mode will enable continuous focusing while the shutter release button is halfway pressed. This means that if you recompose the scene or if the object moves, the camera will focus on what is now in the focus area. The advantage of this mode is that you can press the shutter release anytime as the camera has not locked its focus and exposure. This is very useful when the object you are capturing is constantly on the move such as in sports photography.
Auto select (AF-A or AF-Focus)
This mode is a combination of both AF-S and AF-C. In this mode, the camera will decide which mode is appropriate for the given scene.
The shooting mode button gives you the option of choosing between the various shooting modes. The basic modes available in most DSLRs are Single Frame, Continuous, Self-Timer and Remote. Some higher-end DSLRs come with additional modes such as Continuous High and Low or Delayed and Quick-Response Remote, giving you more control over how the shutter release is activated.
How to Use a DSLR Series: