Camera Metering Modes

Regardless of what shooting mode we use, we rely on the camera’s built-in light meter to achieve proper exposure. In Manual mode we can look at the exposure meter in the viewfinder or on the back LCD screen and adjust settings until the meter indicates proper exposure. In any of the auto exposure modes, the camera will choose settings based on the light meter reading. In either case, the light meter will recommend settings that will achieve a middle tone for the scene or subject. On a black and white scale that middle tone is somewhere around 18% gray.

You can choose whether the camera evaluates the entire scene or just a part of it by changing the metering mode setting. There are three main metering modes that are useful for forensic work. They are Matrix/Evaluative metering, Center-Weighted metering, and Spot metering. Each mode will evaluate the scene differently. As a result, different metering modes can result in very different exposures. To get the best results we need to be familiar with how each mode works, when they are likely to be accurate, when they may be fooled, and how to make adjustments if we are not getting the exposure that we want.

The 1983 Nikon FA was the first camera with a smart light metering system. The FA's Advanced Multi-Pattern (AMP) metering took meter readings from several locations around the frame and  used a computer algorithm to determine exposure settings.

Matrix/Evaluative metering is the most sophisticated light metering system available on most cameras. It provides fast and reliable metering over a broad range of lighting scenarios.  It makes metering simple and easy. It’s what I use most of the time and the results are usually very good.

As good as Matrix metering is, there are scenarios where it may not give the best exposure. These situations include strongly backlit scenes, scenes with a lot of bright sky, scenes with large areas that are in shadow, or subjects that are darkly colored. In these cases switching to Spot or Center Weighted metering may get better results. These metering modes measure light from a small area of the scene allowing the shooter to choose what part of the scene to meter.

Spot metering is the most precise metering mode. This metering mode measures the light reflecting off a tiny portion (usually 1-5%) of the frame.  It allows the photog to decide exactly what in the composition to expose for. Spot metering can be very useful for challenging lighting scenarios. These include high contrast and backlit scenes. The downside to spot metering is that you have to be very careful where you place the metering point. You will typically get the best exposure if the object you meter from is of medium brightness around that 18% area on the gray scale. If the object is much brighter or much darker than that, the resulting photo may be under or over exposed. 

Metering Example - Pole-Mounted Transformer 

Matrix metering works well in most situations, but not all. Above is an example of a common situation where Matrix Metering leads to a poor exposure. The background of this photo is a bright sky, which is backlighting the subject. Matrix metering responds to the brighter than normal background by turning down the overall exposure. While this provides a good exposure for the sky, our subject is severely underexposed.  For the second photo I switched to spot metering and placed the focus/metering point on the transformer. The spot meter exposed the transformer for a middle (18%) gray. 

Using Spot Metering

On many cameras the photographer can use a keypad or joystick on the back of the camera to adjust the focus/metering point where they want it within the frame. If your camera allows for this, move the focus/metering point to the location in the frame from which you want to meter. Once this is set, press the shutter button and take the photo. In some cameras and under certain settings, the focus/metering point will be fixed in the center of the frame. If that is the case, point your camera so that the subject you want to expose for is under the focus/metering point in the very center of the frame. If the composition looks good, press the shutter button and take the shot. If you don’t want the focus/meter point in the center of your composition then you will want to focus and recompose. To accomplish this, point the camera at the object you want to meter from just as before, but this time, instead of pressing the shutter button all the way, press the shutter button halfway and hold. Holding the shutter at the halfway point will lock the exposure and the focus point on your target. While still holding the shutter button halfway, recompose your shot the way you want it and press the button the rest of the way.*

Gray Cards

Sometimes professional photographers will use spot meters in combination with a gray card for metering tricky exposures. This is nothing more than a piece of cardboard, plastic, or fabric that's 18% gray. To use a gray card, position the card so that it is under the same lighting as your subject. Point the spot meter at the card, take the meter reading, and lock in the exposure. Gray cards are seldom used in the field but it's helpful to understand the technique. You can use middle tone objects or areas of your composition to spot meter from, just as you would do from a gray card. Green grass works well for this as does the palm of your hand. Some forensic scales come in 18% gray making it possible to use the scale for exposure reference for your macro shots. The key is to make sure that the thing you are metering off of is in the same light as your subject. For example, if your subject is in direct sunlight don't meter off something in the shade. 

While spot metering can be very useful in certain circumstances, it can also be a bit finicky to use. Slight changes in meter point placement can have a big impact on exposure. If the point from which the spot meter reading is taken is darker or brighter than average, the resulting exposure can be off. Center weighted metering makes it a bit easier to properly expose a subject in the center of the frame. This metering mode concentrates most of the exposure calculation on the center area of the frame (where photographic subjects are most often located) while giving less weight to the edges of the composition. 

You can use Center Weighted metering like you would Spot metering, either by taking the meter reading from the very center of the composition or by focusing and recomposing. Center Weighted metering is more forgiving than spot metering.  Because a center weighted meter reading is taken from a larger area, small shadows and highlights will not change the exposure dramatically like they can in spot metering mode. 

Metering Mode Example 1 - Building against a bright sky

Example 2 - Exhaust fans

Example 3 - Weather head against a bright sky

Example 4 - Evidence against a dark background

Conclusion

When used properly, any metering mode can be used to achieve good exposure. The key is understanding how the modes work and how to use them to get the best effect. The best way to learn this is by experimenting. Exterior building shots are great for practicing with exposure modes especially if the building is backlit. If your exposure turns out too dark or too bright, try moving the focus/metering point. Try the focus/meter and recompose technique. Try switching to a different metering mode.

In the vast majority of cases one of these techniques will get your exposure where you want it to be. But there are times when your camera's automatic and priority modes just won’t give you the exposure you’re looking for. This may happen in scenes that are much brighter than normal (bright snowy days) or scenes with lots of dark-colored areas (fire debris). For those instances, we may need to adjust the exposure manually. We’ll take a look at when and how to do this in the next post where we will discuss Exposure Compensation.