METERING WITH YOUR CAMERA
Getting consistently correct, or should I more rightly say "intended" exposure, is one of the common challenges that a photographer faces. Problems in this area often come about, simply as a result of metering incorrectly. It's very easy to get wrapped up in shutter speed, aperture and ISO but, what metering mode the camera is in, often doesn't enter the equation for many people. It's just a button on the camera that they've never fiddled with, because they don't know what it does.
So what exactly is a metering mode? In essence, it's how your camera measures the amount of light in the frame, to then calculate what it perceives as correct exposure. There are different ways it can do this and the number can vary from camera to camera. For the most part there are at least 3 on modern DSLRs. These are:
Spot Metering - Only the area around the selected AF point is measured for colour and brightness
Center-weighted - As the name suggests, the camera puts emphasis on the middle area of the frame. This area can be increased by varying amounts on different camera bodies. A D90 has 6, 8 and 10mm options, whereas the D800 has 8, 12, 15, 20mm and average which measures the entire frame
Matrix - is similar to average meter except that it divides a wide portion of the frame into multiple segments. It then bases exposure a variety of information, which again includes brightness and color.
The most commonly used mode is Matrix (Evaluative as its also known) and it generally does a pretty good job of managing exposure for you. There are times though when you need to take control of exposure because it's not delivering the desired results.
For example, on a summer's evening you decide to go to a romantic location with ocean views with your partner, for a picnic. Towards the end of the meal you break out the camera to take a photo of your significant other. The Sun is setting behind them and providing beautiful warm tones and gently rim lighting them. In matrix mode the resulting image will be a silhouetted figure on a sunset sky. Although this has its artistic merits, it's not what we ideally want here. The camera is reading all the information in the frame and knows it's going to need either a narrow aperture (f/16, f/22, etc.) or a fast shutter speed to avoid over exposure of the scene.
If instead we change the metering mode in camera to spot, the camera will instead only read the amount of light reflecting from our subjects face (provided we have the AF point on it) and select an aperture/shutter speed to suit. We'll then have correct exposure. Obviously this may result in motion blur, since their face will be dark due to the fact no light is hitting it directly. If the shutter speed is too low, we'd need to compensate by upping the ISO (in the absence of having auto ISO enabled, which I don't like doing personally). If we were prudent enough to bring a small reflector with us, we could bounce some of the ambient light coming from behind, back into the subject's face or even opt for a bit of fill flash to balance out the exposure.
A trick I sometimes use is to put the camera into aperture priority mode (A) on Nikons, (Av) on Canon cameras. I then zoom in and fill the frame completely with my subject's face and take a picture. After this I review the picture on the camera, making sure there are no exposure issues. If all is well, I make a note of the shutter speed in the EXIF data and switch the camera to Manual mode (M) (not manual focus) and dial in the same aperture as before, along with the shutter speed from the previous photo. By doing the same, you should now have correct exposure on your subject's face, regardless of what the background is doing. This allows you to frame your subject however you like, without fear of the meter being tricked from the bright sunset behind and resulting in a silhouette. Provided they keep their back to the Sun, or at least don't face into it, you should have good exposure, all the while the light remains constant. Just be warned that as the Sun sets, the available light will diminish and you'll have to meter again to avoid underexposure.
So while our cameras generally do a pretty decent job of getting correct exposure, they aren't infallible. They don't know what we are intending to do with an image and so we have to input that information ourselves. Understanding what metering mode to use and when, is a vital part of getting the images we want, so make sure you learn the differences.