What you’ll learn in this chapter:
1. Basics of In-Camera Metering
2. When and How to Use Different Metering Modes
3. How to Understand Exposure Histograms
The camera’s built-in metering system can handle everything for me, so I just need to shoot using the exposure settings it provides, right?
Aperture, shutter speed, and ISO are the three key elements that determine exposure, and the camera’s metering system matches them together. The camera’s metering system simply measures the total amount of light reflected from the scene — known as “reflective metering” — and then determines a set of exposure parameters to achieve a balanced exposure. You can choose the fully automatic mode and let the camera decide aperture, shutter speed, and ISO. Alternatively, you can also manually set these parameters yourself.
Okay, reflective metering... should I be worried about that?
Yes, because there are many situations during metering that can lead to measurement errors. Most cameras’ metering systems determine what they consider to be a “balanced” exposure by aiming to make as much of the scene as possible appear as 18% mid-level gray. This is because, generally speaking, most scenes reflect around 18% of the incoming light. For many scenes, this default criterion works well — such as lush green grass under a blue sky. But in reality, our world isn’t entirely made up of mid-tones. Dark forests, bright snow scenes, and white skies, etc., can all confuse the existing metering systems. That’s why we often encounter exposure issues, resulting in photos that look either too dark or too bright.
So, the camera’s metering system doesn’t “know” what the subject it’s facing actually looks like, right?
The issue lies in the proportion of a certain brightness level within the frame. If you’re photographing a swan in the snow or a tree in a pine forest, the metering system treats it no differently from a normal scene — it still wants the overall exposure of the scene to be as close to 18% mid-level gray as possible. Take the example of a swan in the snow: with a white background and a white subject, the scene is very bright, and a lot of reflected light enters the lens. But the camera’s metering system doesn’t realize this — it still assumes the scene itself should be mid-tone, just being hit by extremely bright light. As a result, it decides to reduce the exposure in order to make the “overly bright mid-tone” scene appear close to 18% mid-level gray. The outcome is that the photo becomes underexposed, and the snow looks gray. Similarly, if the scene is almost entirely dark, such as in the previously mentioned forest example: such a scene reflects very little light, so the metering system still treats it as a mid-tone scene — just one that’s lit by weaker light. So the metering system increases the exposure to raise the overall brightness of the image to 18% gray. And the final result is an overexposed photo where the trees look pale.
So, how should you deal with these exposure errors?
This is where exposure compensation comes in. By pressing the exposure compensation button marked with “+/-” and turning the camera’s control dial left or right, you can correct the exposure selected by the metering system.
On the camera’s viewfinder and display screen (top LCD or rear screen), there is an electronic simulated exposure display — a straight line divided into multiple stops. There is a marker at the midpoint of the line, which represents the exposure that the camera’s metering system considers “correct.” When using exposure compensation, as you adjust the compensation value by turning the dial, you’ll see the indicator marker on the electronic simulated exposure display move accordingly in the positive or negative direction. When it moves toward the “+” sign, the photo becomes brighter; when you turn the dial in the opposite direction, the marker moves toward the “-” sign, making the photo darker.
When looking at the scene through the optical viewfinder, you can’t directly see the effect of these adjustments on the image. However, if you’re using live view mode or an electronic viewfinder (EVF), you can observe the effect of exposure compensation in real time. Exactly how much compensation is needed depends on multiple factors, such as the tonal range of the subject, lighting conditions, and the metering mode used (see below).
What are metering modes?
Most digital SLR cameras offer the following three basic metering modes: evaluative (or matrix) metering, center-weighted metering, and spot metering (these modes may have different names on different cameras). These metering modes emphasize certain areas of the scene to varying degrees, thus giving the camera’s exposure results a certain bias. If you want to meter a specific area of the frame precisely, spot metering is the best choice. As the name suggests, it meters only a very small area of the frame (typically the exact center of the viewfinder, but many cameras also allow you to use the currently active autofocus point as the metering point).
Center-weighted metering is a mode inherited from film cameras. Although it meters the entire frame, it assumes that the main subject is located near the center of the frame, so it places more emphasis on the central area. Modern DSLRs also feature intelligent evaluative or multi-zone metering systems (such as Canon’s Evaluative Metering and Nikon’s Matrix Metering), which divide the frame into multiple zones, measure the brightness of each zone separately, and then analyze and integrate the results before applying them to the exposure.
When you half-press the shutter, the camera identifies, measures, and calculates not only the amount of reflected light but also many other factors: the position of the autofocus point, the size and distance of the subject, the color of the scene, and whether there’s backlighting, among others.
How much exposure compensation should I apply?
One issue with evaluative metering is that when the camera displays its final recommended exposure settings, you don’t know whether the camera system has made adjustments, or to what extent. Let’s revisit the swan in the snow scenario: the advanced evaluative metering system may have recognized that it’s dealing with an all-white scene and thus decided to increase exposure directly to ensure the image is bright and snowy.
But if you’re unaware of this and still apply positive exposure compensation (because you think the camera’s metering system will treat the snow scene as 18% gray), the photo may become overexposed. On the other hand, the camera might not have increased the exposure enough, so if you don’t apply any positive compensation, the photo could end up underexposed. This is why it’s important to familiarize yourself with your camera’s “behavior” — understanding how its evaluative metering system typically reacts under different scenes and lighting conditions, and whether it has any tendencies. This way, you can anticipate in advance whether the current scene might cause the metering system to “misjudge.”
If a scene contains midtones, bright highlights, and very dark shadows, how should I expose it?
There’s more than one way to solve this problem, but one thing is certain: blown-out highlights that are pure white with no detail are very unappealing and can disrupt the overall image. As for the shadow areas, even if they’re slightly underexposed, you can often recover a lot of detail during post-processing.
Therefore, we generally follow the principle of “getting the highlights right.” That means we need to ensure that the right side of the photo’s exposure histogram doesn’t show any “clipping” (exceeding the display range of the histogram). Trust us—“crushed” black shadows are nowhere near as bad as “blown-out” white highlights.
Manual Exposure
If you use your camera’s manual exposure mode, then you’re in full control of the entire exposure process. The metering system is still active and will provide feedback on your current exposure settings through the viewfinder and the electronic analog exposure display on the screen.
The midpoint of the electronic analog exposure display’s indicator line represents the correct exposure for midtones. The three stops to the left and right of this midpoint also fall within this exposure range. The exposure indicator mark moves left and right along this line to show how much the subject’s brightness, according to the camera’s metering, deviates from the midtone level. Note that the “midtone” here is determined by the camera based on its own metering results—the camera system doesn’t know what object you’re actually pointing at.
Aim the lens at a midtone area—the exposure indicator mark should be near the midpoint of the indicator line. If it isn’t, adjust the exposure parameters accordingly.
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