More on exposure


Metering modes

In an earlier article, you learnt that the camera has a built-in light meter that helps you expose the scene properly by placing a solid bar under 0 if it thinks that the image is properly exposed, on the + side if the image is overexposed, and on the – side if the image is underexposed. Modern dSLR cameras have multiple metering modes.

- Evaluative metering: this is the most technologically advanced mode available in cameras today. The camera divides the scene into multiple zones, and gives each one a different weight based on color and light intensity, placement in the frame, and other proprietary considerations when outputting the light meter reading. This is the default mode on most cameras, and works fine for most subjects.

- Partial metering: instead of examining the entire scene when outputting the light meter reading, the camera considers only the area under a circle in the middle of the frame; for Canon cameras, this is 8% of the frame.

- Spot metering: this is similar to partial metering, but the metering area is a tiny circle in the middle of the frame that covers only 3.5% of the frame. Use this when it’s absolutely necessary to get the exposure correct for one aspect of the photo. For example, if you have a very bright object such as the sun in the frame, you can use spot metering to exclude it from light meter calculations so that all other elements in the photo are correctly exposed.

- Center-weighted average metering: this mode gives more importance to a circle in the middle of the frame, but also considers the rest of the frame.

Evaluative metering

Partial metering

Spot metering

Center-weighted average metering

When using Av or Tv modes, if you notice that your camera’s algorithm is always under or overexposing pictures, you can use exposure compensation in the opposite direction.

You can’t trust the LCD

After taking a photo, most people will instinctively look at the camera’s LCD to see the picture that they’ve just captured. The problem is that the screen brightness will vary with the ambient light, often leading to a false sense of achievement in the field, and a subsequent disappointment when you’re later editing the photo at home in a properly balanced light environment. The solution is to look at the histogram on the LCD instead of the photo. The sample histogram below was taken from a photo loaded in Adobe Photoshop, but you’ll see something similar on your camera’s LCD.

Each pixel in a photo has a luminosity value, and that is shown on the histogram’s x-axis. Entries on the left denote shadow areas, those on the right denote highlight areas, and those in the middle are midtones. On the extreme left side are pixels that are absolute black, and on the extreme right side are pixels that are absolute white. More often than not, if the histogram shows pixels at the extreme right side, you have overexposed parts of the photo, which will appear to be white, having lost all color. The y-axis shows the number of pixels at a particular luminosity value.

Modern cameras can show you per-channel histograms in addition to the luminosity histogram; viewing separate red, green, and blue channels is very important because you want to spot trouble colors in the field where you can still modify your exposure settings to fix the problem.

Expose to the right

Notice that the histogram above leans to the right, so there’re more bright areas in this photo than there are dark. I’ll cover sensor noise in a separate article, but for now, remember that dark areas of the photo have the largest amount of noise. Ideally, you want to ensure that you don’t blow the highlights, i.e., you don’t want any pixels on the extreme right side of the histogram, but if you have a bright object such as the sun in the frame, this will be unavoidable. At the same time, you want your histogram to have more peaks in the right half than in the left, to avoid noise.

Highlight clipping

Always enable highlight warning in your camera; now, if your photo has areas where some pixels correspond to the extreme right side of the histogram, those areas will blink in the photo on the LCD. If it’s the sun that is blinking, there’s not much you can do, but if it’s some other crucial section of the photo that you have overexposed, you can immediately take another photo with the correct exposure settings.

Blown-out skies

Now that you have a dSLR, you must never take another photo with a white sky! Your eyes are magnificently engineered to allow you to see light and dark areas equally well at the same time, but cameras are a little less sophisticated. If you simply dial in exposure settings to bring the light meter reading to 0, and release the shutter, it’s quite possible that your foreground will be properly exposed, but the sky will have turned out white. The reason is that foregrounds are often darker than the sky, and the camera’s evaluative or center-weighted metering algorithm figures that the foreground is probably more important than the sky.

To avoid this problem, always point your camera to the sky when taking a meter reading, excluding all other areas of the photo. Once you have dialed in the correct exposure settings, recompose the shot to include your original subject; you’ll notice that the meter reading is no longer 0, but this is fine. You’re now guaranteed to get a properly exposed sky, but your foreground may be underexposed. There’re two solutions here:

- If your foreground includes a human, the face may turn out quite dark because you’re now exposing for the sky instead of the subject’s skin tone. Either move the person, reflect light on to him/her with a reflector disc, or use fill flash. Yes, flash. People normally use flash at night, but in fact, the best time to use it is during the day, not at night! I’ll cover this in a separate article.

- If you’re taking a photo of the Grand Canyon at dusk, you’ll notice that the sky is properly exposed, but the foreground, and the canyon are both a large area of darkness in your photo. In this case, you still need to expose for the sky, but you now need special filters, which are the subject of another article.

ID: more exposure