Photography: exposure and metering

Want to take pictures like this?

Waves of Nacre

Waves of Nacre

Mediterranean Sunset

Golden Leaves

See The Light

Then you’ll need to master the art of exposure – controlling the amount of light which enters your camera.

 

What is exposure?

Like the eye, cameras operate by recording light when it strikes a sensor in the camera. The camera’s sensor is made of a grid of semiconducting material (the number of light-sensing “buckets” in this grid determine the resolution of the photograph). When light strikes a “bucket”, the incoming energy frees paired electrons in the material, causing an electric current to flow. This electricity is picked up by a processor in the camera (its “brain”) and the responses from the “buckets” are compiled to form an image.

The amount of light striking the sensor while the camera’s shutter is open determines how much light is recorded in the image. In other words, the more light that enters the camera (or the longer the camera is exposed to a constant source of light), the brighter the resulting image will be. This is called the exposure.

Camera’s metering

Underexposed

Overexposed

Factors Controlling Exposure:

Aperture:

In order to control the amount of light entering the camera (as well as to focus), the camera filters the light through a hole, like the pupil of an eye, before it strikes the sensor. This hole is called the aperture. The “wider” (more open) the aperture, the more light can enter the sensor and expose the photograph. Widening the aperture therefore has the effect of making a photograph brighter, all other things being equal. (Since the aperture is also used to focus, widening the aperture also makes the area in focus smaller, blurring more of the background or “reducing the depth of field”).

Aperture width is measured in f stops. Lower f numbers represent wider apertures; for example, a lens set to f/2.8 would admit much more light in a fixed span of time than the same lens set to f/22.

“Golden Leaves” was shot at f/4.5. As a result, the background was blurred out behind the leaves, creating a very visually interesting texture and contrast of colors.

Shutter time:

Most of the time, photographers don’t leave the shutter open for a fixed amount of time, however. It is most common to fix the shutter time when taking photos of moving scenes, such as waterfalls or highways, in order to control how much the motion “blurs”. The camera is capturing all light that enters the sensor while the shutter is open. As a result, photos with short shutter times, such as “Waves of Nacre” above, show instantaneous moments in time, while moving elements in photos with long shutter times will blur the motion. Notice how you can see the individual drops in the water – if the shutter time was longer, you would instead see a more continuous stream of water, since the camera recorded the image for a longer period of time. This can be desirable or undesirable, and is the reason why night and indoor photographers often carry tripods (everyone’s hands shake sufficiently to move the camera around and make the entire photo blurry if it has a long exposure time).

The amount of light passing through the aperture for the duration of the exposure time determines the exposure of the photo; that is, how bright or dark it will be.

“Waves of Nacre” was taken with a short (1/640th of a second) shutter time. Note how the individual droplets of water are visible as the wave crashes against the shore.

Niagara Falls, taken with a long shutter time (1 second). Note how the water of the falls blurs. The camera was on a tripod to avoid hand shaking, which would ruin the shot.

ISO:

There is also another variable called ISO. This is the electrical amplification provided to the camera’s sensor. Raising it can allow the camera to take brighter shots with the same amount of light in the same amount of time. However, this comes at a cost: camera sensors are “noisy”, recording random low-level amplification (as well as low-level responses to wavelengths of light that the human eye cannot see). Increasing the electrical amplification also amplifies this noise, resulting in pictures that can be grainy or contain colored “spots”. Still, raising the ISO is often acceptable in low light situations, because the alternative may be getting blurry shots or no shot at all.

Metering

Underexposed photos will be too dark to see and overexposed photos will be washed out with white, while properly exposed photos will look similar to what the eye sees. All modern digital cameras have built in sensors and algorithms which determine the proper level of exposure. This is called metering. Based on the camera’s guess of the correct exposure, the camera can automatically set its aperture, shutter time, and ISO to appropriate values. It is also common for more advanced photographers to manually adjust one or two variables (usually the aperture and ISO) and let the camera automatically handle the third. Though relatively uncommon, some photographers will shoot in “full manual” mode, where they are completely responsible for manually metering the camera by setting all three values.

There are several metering modes on a high-end digital camera, but the most important are evaluative metering (also called matrix metering) and spot metering. Evaluative metering tries to keep as much of the scene properly exposed as possible, but gives some weight to the center of the image, as the camera is usually pointed directly at the most interesting part of the scene. Spot metering ignores the periphery of the image altogether, and focuses on exposing only the center. It is extremely useful in high-contrast scenes, where the background may be much brighter or darker than the subject. For example, “See The Light” above was taken using spot metering mode, because the plant was illuminated while the surrounding areas was not. If I had shot the photo in evaluative mode, the camera would have attempted to illuminate the dirt, completely washing the sunlit plant out.

Spot metering ensures that the camera tries to keep only the plant in the center of the image exposed, rather than attempting to expose the dark background above it (which would cause the plant to appear too bright and washed out)

One more thing – since all scenes contain mixtures of light and dark areas, cameras sometimes guess wrong, or try to expose a part of the scene that you’re not interested in. When this happens, it’s time for the human to step in – cameras have a setting called exposure compensation which allows you to step the exposure that the camera meters up or down by a fixed number of stops. This allows you to fine tune the brightness of the scene without having to shoot in full manual mode.

Happy snapping!

For more of Michael’s photography, visit his Behance portfolio or email him.

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About Michael Barnathan

Michael Barnathan is a computer scientist, inventor, educator, and semiprofessional photographer. He is also the founder of Project Polymath, something which he left a Senior Software Engineer position at Google to pursue. He holds a Ph. D. in Computer and Information Sciences from Temple University. When he isn't busy with the complexities of starting a university, he enjoys inventing devices such as EEG helmets, raising the survival rate for breast cancer through diagnostic computer algorithms, composing music, and birdwatching.