Pixiq

Just maybe the most important development of digital photography has been the histogram. Until its arrival, photography, even with exposure meters, left you in the dark about knowing if you had truly succeeded in taking a well-exposed picture until you saw your developed film. Before the histogram, taking a picture was a bit like hitting a blind golf shot to a green that’s over a hill and nestled in a wooded nook. Thwack–it feels and looks like the perfect shot, but until you run to the crest of the hill you won’t know. Now, with the histogram, you can know instantly.

Let's take a quick look at exposure: It is the amount of light the camera delivers to the sensor to create a picture of the proper brightness. Why is it so important? Because you need to properly expose the scene in order to achieve full dynamic range of the sensor and capture continuous subtle tones, colors, and details from shadows to highlights. The camera uses the size of the lens opening (aperture) and duration of the shutter speed to control that amount of light. It’s that simple. If your exposure is off, your quality is off. Maybe just a nick off, maybe even unnoticeable (and maybe not), but still it’s off.

First you should meter the scene accurately to correctly expose it. To do this, you need to determine and meter both the darkest area that should show shadow detail and the lightest area that should show highlight detail. The camera does this with a built-in exposure meter. This meter sees the world as midtone objects, things that reflect an average amount of light. Midtones are middle grays. Green grass is a midtone, and so are faded Levi’s. A meter wants to make everything into midtones. If it sees a sunlit field of snow, it thinks midtones. If it sees freshly laid asphalt, it thinks midtones, even though neither of these scenes is gray.

Determining Exposure

But today’s cameras are smart (used to be the photographer had to be smart) and use advanced exposing systems to compensate for the middle gray leanings of the exposure meter. Multi-segment (also known as matrix or honeycomb, etc.) metering is an intelligent exposing system. It uses a computer program to evaluate the brightness coming from many segments of the scene—the dark parts, the bright parts, the “important” parts, and so on, in an attempt to give good exposure. Its program includes hundreds, if not thousands, of likely scene reflectances and compares the scene you are photographing to those in its memory, adjusting exposure accordingly. This type of metering works pretty darn good most of the time (but do note that it is not always the best to determine the exposures required for the shadow and highlight areas—that is where spot metering, the histogram, and the determination of dynamic range come in—see below).

Achieving correct exposure is the result of using the histogram in conjunction with the camera's meter. The meter tells you what to do. The histogram tells you whether you succeeded. Now, if I were a cynic (and I am) or if I were excessively practical (and I am), I would tell you to forget what I just said about metering and simply take a few test pictures, looking at the histogram to determine if you’re getting the right exposure. Because in the end, you evaluate the histogram and confirm that your exposure is maximizing the dynamic range. Hooray for the histogram. It is our exposure savior.

Using the Histogram to Determine Exposure

Keep in mind, determining exposure by histogram is only effective when you photograph immobile subjects that allow enough time to figure out the best exposure by shooting and reshooting the scene. Simply take the picture and see what the histogram reveals. If you understand the histogram, and have an inkling of scene reflectance, you can create a good exposure.

Here are the steps to take:

1. Set the aperture to achieve the depth of field you want. Or, if you think shutter speed should take priority, set the shutter speed you want.

2. Let your camera’s built-in meter set exposure, then take a picture.

3. Look at the histogram. If it’s too far to the left, you need to increase exposure (larger aperture or slower shutter speed). If it’s too far to the right, you need to decrease exposure (smaller aperture or faster shutter speed).

4. To change the exposure, use the exposure compensation dial (or by changing the aperture or shutter speed if you’ve set the camera to Manual mode).

5. Shoot successive picutures, repeating steps 3 and 4 until you achieve a histogram displaying the full range of tonal values without any clipping.

6. You may want to increase exposure by 1/2 stop and take a second picture so the brightest highlights are slightly clipped (overexposed). You can most likely recover them if you record using RAW format. With digital cameras, you should slightly overexpose to maximize the tonal scale.

Determining Dynamic Range

Despite the value of the histogram, it still pays to meter the shadows and highlights to determine the the dynamic range for the scene. That's easy enough to do.

1. Set the camera to Aperture Priority mode select the aperture you want to use.

2. Set the metering mode to spot if you need to determine exposure of a small subject in the distance. Set it to center-weighted averaging if you can walk up to the subject and take a close-up meter reading.

3. Take a meter reading of the brightest area you want to show detail. Note the recommended shutter speed; let’s say 1/30 second.

4. Take a meter reading of the darkest area you want to show detail. Again, note the recommended shutter speed (maybe this time it is 1/2000 second).

5. Determine the dynamic range by counting the full stop intervals between the two readings (see table below).

If the dynamic range exceeds six stops, you probably won’t be able to capture it all in a single exposure, or you will have to compromise and sacrifice highlight or shadow detail. Since our hypothetical photo is six stops, you can probably use one exposure to capture the full dynamic range.