Nearly every digital camera on the market, even most point&shoot models, offer a choice of metering system. This deceptively simple option can completely alter the result of images. Luckily for many compact cameras and a number of mirrorless ones, the LCD or EVF correctly previews the effect of the currently selected metering mode. For DSLR users, it is harder and you need to become familiar with how metering modes behave. Naturally, the question is: When to use a particular metering mode?
Every digital camera has a Multi-Segment metering mode. Nikon calls theirs Matrix. Canon calls theirs Evaluative. They work the same way except that each has its own secret formula. In fact, even different Nikon cameras have different Matrix modes. These modes are generally targeted to the intended users of the particular camera. So a lower-end Nikon tends to produce brighter images compared to a higher-end model under similar conditions. These Multi-Segment modes are very sophisticated and does a good job in most situations.
Spot is used when you know what part of the scene is going to be your midtone, that is the part of the scene that you want to show as 18% luminance which corresponds to a middle tone, hence the term midtone. This is simply based on an old research which concluded that the average scene reflects 18% of light that falls on it.
To use Spot metering, you must point the spot meter at that part and lock the exposure using either the AE-L button or by half-pressing the shutter. Most cameras are setup like this initially but you can change that on certain models. Then, you reframe your subject without releasing the shutter or AE-L button and take your shot. When using the Spot meter, nothing else is considered by the chosen spot.
Many Olympus cameras feature also Highlight Spot and Shadow Spot options. These work similarly to spot except that they meter for a highlight or shadow, respectively. These are generally more convenient to use since it is easier to tell which part of the scene is brightest or darkest, than to tell which one falls in the midtone. Users of other cameras can use Spot in conjunction with Exposure-Compensation to achieve the same exposure. Dialing around -3 EV of EC gives a shadow spot while dialing around +3 EV gives a highlight spot. The exact amount of EC needed depends on the camera’s dynamic-range which is itself dependent on the selected ISO.
Center-weighed is basically the ancestor of multi-segment metering. It tries to make the central part at least 18% bright but will vary the results depending on the brightness of the surrounding areas. Average metering is also simple in that it simply averages several measures which are evenly distributed across the frame.
In most cases, selecting a metering mode to use is easy:
- Use Spot metering when you know which part of the scene should be your midtone.
- Use Multi-Segment metering otherwise.
- Use Center-Weight or Average when Multi-Segment fails.
- Adjust with EC as needed. No metering system is perfect yet!