Getting a Handle on Exposure

Exposure can make or break your photo. Correct exposure brings out the detail in the areas you want to picture, providing the range of tones and colors you need to create the desired image. Poor exposure can cloak important details in shadow, or wash them out in glare-filled featureless expanses of white. However, getting the perfect exposure can be tricky, because digital sensors can't capture all the tones we are able to see. If the range of tones in an image is extensive, embracing both inky black shadows and bright highlights, we often must settle for an exposure that renders most of those tones—but not all—in a way that best suits the photo we want to produce.

There are four things within our control that affect exposure, listed in "chronological" order (that is, as the light moves from the subject to the sensor):

■ Reflected, transmitted, or emitted light. We see and photograph objects by light that is reflected from our subjects, transmitted (say, from translucent objects that are lit from behind), or emitted (by a candle or television screen). When more or less light reaches the lens from the subject, we need to adjust the exposure. This part of the equation is under our control to the extent we can increase the amount of light falling on or passing through the subject (by adding extra light sources or using reflectors), or by pumping up the light that's emitted (by increasing the brightness of the glowing object).

■ Light transmitted by the lens. Not all the illumination that reaches the front of the lens makes it all the way through. Filters can remove some of the light before it enters the lens. Inside the lens barrel is a variable-sized diaphragm called an aperture that dilates and contracts to control the amount of light that enters the lens. You, or the 40D's autoexposure system, can control exposure by varying the size of the aperture. The relative size of the aperture is called the f/stop.

■ Light passing through the shutter. Once light passes through the lens, the amount of time the sensor receives it is determined by the 40D's shutter, which can remain open for as long as 30 seconds (or even longer if you use the Bulb setting) or as briefly as 1/8,000th second.

■ Light captured by the sensor. All the light falling onto the sensor is captured. If the number of photons reaching a particular photosite doesn't pass a set threshold, no information is recorded. Similarly, if too much light illuminates a pixel in the sensor, then the excess isn't recorded or, worse, spills over to contaminate adjacent pixels. We can modify the minimum and maximum number of pixels that contribute to image detail by adjusting the ISO setting. At higher ISOs, the incoming light is amplified to boost the effective sensitivity of the sensor.

These four factors—quantity of light, light passed by the lens, the amount of time the shutter is open, and the sensitivity of the sensor—all work proportionately and reciprocally to produce an exposure. That is, if you double the amount of light, increase the aperture by one stop, make the shutter speed twice as long, or boost the ISO setting 2X, you'll get twice as much exposure. Similarly, you can increase any of these factors while decreasing one of the others by a similar amount to keep the same exposure.

Most commonly, exposure settings are made using the aperture and shutter speed, followed by adjusting the ISO sensitivity if it's not possible to get the preferred exposure (that is, the one that uses the "best" f/stop or shutter speed for the depth-of-field or action-stopping we want). Table 4.1 shows equivalent exposure settings using various shutter speeds and f/stops.

Table 4.1 Equivalent Exposures

Shutter Speed


1/30th second


1/60th second


1/125th second


1/250th second




1/1,000th second


1/2,000th second


1/4,000th second


1/8,000th second


When the 40D is set for P mode, the metering system selects the correct exposure for you automatically, but you can change quickly to an equivalent exposure by holding down the shutter release button halfway ("locking" the current exposure), and then spinning the Main Dial until the desired equivalent exposure combination is displayed. This program shift mode does not work when you're using flash.

In Aperture Priority (Av) and Shutter Priority (Tv) modes, you can change to an equivalent exposure, but only by adjusting either the aperture (the camera chooses the shutter speed) or shutter speed (the camera selects the aperture). I'll cover all these exposure modes later in the chapter.


Throughout this chapter (and throughout this book), if I direct you to use the Quick Control Dial on the back of the camera, you must remember to have activated the dial when you turned on your Canon EOS 40D by rotating the power switch all the way up to the topmost counterclockwise position. Turning it up one position to ON does not activate the Quick Control Dial for functions other than menu navigation. I won't be reminding you to do this every single time, so keep it in mind.


If you're really new to more advanced cameras, you might need to know that the lens aperture, or f/stop, is a ratio, much like a fraction, which is why f/2 is larger than f/4, just as 1/2 is larger than 1/4. However, f/2 is actually four times as large as f/4. (If you remember your high school geometry, you'll know that to double the area of a circle, you multiply its diameter by the square root of two: 1.4.)

Lenses are usually marked with intermediate f/stops that represent a size that's twice as much/half as much as the previous aperture. So, a lens might be marked:

with each larger number representing an aperture that admits half as much light as the one before, as shown in Figure 4.1.

Shutter speeds are actual fractions (of a second), but the numerator is omitted, so that 60, 125, 250, 500, 1,000, and so forth represent 1/60th, 1/125th, 1/250th, 1/500th, and 1/1,000th second. To avoid confusion, Canon uses quotation marks to signify longer exposures: 2", 2"5, 4", and so forth represent 2.0, 2.5, and 4.0-second exposures, respectively.

Figure 4.1 Top row (left to right): f/2, f/2.8, f/4; bottom row, f/5.6, f/8, f11.

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