What Is Exposure in the World of dSLR Cameras?

The word exposure is thrown around a lot in photography. In the old days, people referred to photos themselves as exposures because you exposed film in the camera to light when you took a picture. Today, most people use terms like shots, photos, images, or even files to refer to their pictures.

Exposure is still a pretty important word in digital single-lens-reflex (dSLR) photography, even though most of us don’t call our photos exposures. I want to dispense with the technical details, for the moment, and bring it down to one very important but simple point:

Exposure is about taking photos and shooting movies that are as dark or as light as you want them to be.

Simple, right? Keeping that straightforward goal in mind puts the details of this article in context. What may be confusing at times is learning the different pieces to the exposure puzzle. Exposure involves:

• Monitoring and evaluating: The camera evaluates the scene’s brightness and suggests exposure settings to capture the best photo. This is called the standard exposure.
• Metering: The camera needs to measure the light in the scene. This is called metering.
• Reviewing: Reviewing photos and looking at their histograms helps you understand what is happening with the camera and where you may need to change settings.
• Troubleshooting: There are several ways to troubleshoot. They include techniques like switching metering modes, using your camera’s manual shooting mode, dialing in exposure compensation, using exposure brackets, and using autoexposure (AE) lock.

You can take more or less control over most parts of the process.

Evaluate exposure

As you evaluate exposure, the point is to assess whether your photos and movies look the way you want them to. You have only three options: They can be underexposed, exposed just right, or overexposed. The photos below show examples of each.

• Underexposed: Unacceptably dark photos and movies are underexposed. For whatever reason, the image sensor captures too little light. Underexposed photos and movies often have clipped shadows — these are dark areas that turn black and lose all detail.

• Just right: When everything comes together and the camera captures just the right amount of light to produce a good shot; that shot has been properly exposed. In general, your goal will be to capture shots that look good — not too dark and not too bright. Exceptions to this idea exist, of course, but shots normally have a “sweet spot” that produces the best photos and movies most of the time.

• Overexposed: Photos and movies that have been overexposed and are too bright. Overexposed photos and movies often have clipped or blown highlights. When this happens, the pixel turns pure white. There are no details at all.

The amount of light that can cause a photo or movie to be under- or overexposed can be a little or a lot. It depends on the scene and camera. If the exposure is off by a relatively small margin, you may be able to fix it using photo-editing software.

Control exposure

Your camera has several shooting modes, also called exposure modes, that determine whether you or the camera control the various exposure settings, such as the shutter speed. This figure shows the Mode dial on a consumer-level dSLR.

Autoexposure modes put the camera in charge of setting the exposure controls. The type of input you have depends on the mode you’ve selected. Basic autoexposure modes require no input from you. Set it and forget it.

Advanced autoexposure modes like aperture-priority or shutter-priority let you set the lens aperture or shutter speed. The camera takes care of the other controls and adjusts them for the best exposure.

You have two options if you want manual control over your camera’s exposure controls: Manual (M) and Bulb (B) modes. If your Bulb mode isn’t on the mode dial, enter Manual mode and lengthen the shutter speed until you see it. The camera still evaluates the scene and displays what it thinks the best exposure is, but you’re responsible for setting all the controls. Although this responsibility may sound intimidating, it gives you the ability to direct the camera to do your bidding.

In addition to exposure modes, your camera has three exposure controls, each with its own unique personality: the aperture of the lens; the shutter speed; and ISO speed. You can also modify the exposure by using a flash or filters.

Compare exposure intervals

There are two ways to describe exposure values and intervals: stops and exposure value (EV). They are different terms that basically mean the same thing. Here’s what you need to know about each:

• Stops are a traditional way of describing exposure intervals. They are mechanical. The term comes from how film photographers change apertures and shutter speeds on their film cameras. They widen the aperture by a physical stop on the lens or make the shutter speed faster by turning a knob to the next stop. This action doubles or halves the amount of light that the film is exposed to. The term, therefore, made its way into the lexicon of photography as a way to double or halve light.

• Exposure value is a measurement of exposure. An interval of 1 EV is synonymous with a stop. Each interval of 1 EV doubles or halves the exposure. You can change the EV by altering the camera’s exposure settings (aperture, shutters speed, or ISO). For example, raising the ISO by a stop increases the EV by +1. Intervals between exposure values are most often measured in thirds and whole numbers. You can often set your camera to control exposure level increments in halves.

This figure shows the relationship between newer camera settings and stops. Each numbered division represents a stop of exposure, which is the same as on the older shutter speed dial. Each stop is divided into thirds by smaller, unnumbered marks. The difference between each stop represents 1 EV. The smaller increments are separated by 1/3 EV.

There are creative and practical differences between a stop of shutter speed and aperture, but in terms of exposure, every stop is equal.