by David Tong

© David Tong

Almost all cameras these days have an Auto-mode, even pro-level cameras will have at least a Program mode which is essentially Auto-mode with flexibility. It’s safe to assume that modern cameras can analyze a scene pretty well to give you perfect exposures every single time, but in reality, they don’t – why?

In many cases, especially day-lit, outdoor scenes, the camera’s automatic modes will give you accurate, instantly usable exposures like the photo on the right. The shot was taken in Program mode with only ISO setting as my user-altered parameter. The image had great exposure and ample depth-of-field.

The problem is, we don’t always get a nice, sunny day with good lighting and balanced contrast everyday. We have to contend with gray skies, white snow, dark night scenes, back and front lit subjects, and so forth. The camera doesn’t know where your subject is standing, neither will the camera know how light or dark a scene should be in real life, the camera’s meter will always do one thing – ASSUME MID-TONE.

Irregardless of what metering mode you select in your camera (evaluative/matrix, center partial, average, spot, etc.) the camera’s reflective meter still assumes one thing, the area where it is metering is neutral mid-tone, or roughly 12% gray.

(Reflective reading = how much light is reflected from the subject back to the camera’s meter)

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© David Tong

That means, the area where you point the camera to, the camera will assume that’s mid-tone. The only difference between an evaluative meter setting versus a spot meter setting is how much of the scene you want to take account for. The smaller the metering zone, the more accurate the user has to be to determine which area of the scene is mid-tone as you’re taking the decision away from the camera’s average reading in wider metering modes.

So what do all these mean? What shooting mode should you use and when should you not rely on the camera’s meter? In general, if you use manual shooting mode in your camera, you’ve pretty much told the camera’s meter to take a back seat and let you do the thinking (which is a good thing as you should be smarter than a small metering chip), but in most cases where we prefer to use any of the other shooting modes such as Tv, Av, or P modes, most cameras will allow you to use the exposure compensation function to override the camera’s chosen exposure.

Try this experiment at home.

Take a white sheet of paper, a black sheet of paper, and a gray sheet of paper (or fabric, whatever), turn off your auto-focus (it won’t make a difference anyway) and take one shot of each piece of paper under the same lighting. You’ll notice that all three images will look gray, and when you review the histogram, the result will form a peak around the middle of your display. That’s because the camera can’t see anything but gray, it’ll simply try to turn anything to that tonal range of 12% reflectance.

Here are some real-world examples that’ll illustrate the usage of exposure compensation.

Figure 1: Auto-exposure, 0 Exposure Compensation

The image above is dominated by my friend’s white shirt. The camera then tries to turn that into mid-tone gray, resulting to underexposure of the scene as evident in the histogram. Note how empty the right-side of the histogram is, indicating the absence of data for white tones.

Figure 2: Auto-Exposure +1 Exposure Compensation

Using the same exposure setting in Program mode, I added +1 exposure compensation and instantly, the image is brighter and the exposure is more accurate compared to the real scene. The histogram shows bulk of the data are on the light side (right), which is the correct interpretation of the scene. Note that the camera slowed down the shutter speed by a stop (1/200) vs the previous image at 1/400sec as we instructed the camera to add 1 stop of exposure compensation previously.

Conversely, this is what happens to a predominantly dark image.

Figure 3: Auto-Exposure, 0 Exposure Compensation

This time, majority of the image is covered with the black shirt (lucky that both of my friends were wearing black and white today, LOL), the camera’s meter is turning black to gray as evident in the histogram, which shows most of the data near the middle area of the graph. The histogram should be biased to the left (dark) side of the graph as the black shirt covers over 80% of the scene.

Figure 4: Auto-Exposure, -1 Exposure Compensation

After setting a -1 exposure compensation on the same shot, the camera sped up the shutter speed by a stop to 1/15 (from 1/8sec in Figure 3), the result is the shirt turned a lot darker and is closer to black. Given the example above, I should’ve used -1 2/3EV to turn the shirt a lot “blacker”, but for the sake of uniformity, I decided to keep the settings at +1 and -1 for both examples. Regardless, the new histogram shows that the image is predominantly darker than gray, as opposed to mostly gray depicted in Figure 3.

We’ll discuss about shooting high-contrast scenes where it’s harder to determine mid-tone in a future article.