How the Eos D Calculates Exposure

Your Canon EOS 40D calculates exposure by measuring the light that passes through the lens and is bounced up by the mirror to sensors located near the focusing surface, using a pattern you can select (more on that later) and based on the assumption that each area being measured reflects about the same amount of light as a neutral gray card with 18 percent reflectance. That assumption is necessary, because different subjects reflect different amounts of light. In a photo containing a white cat and a dark gray cat, the white cat might reflect five times as much light as the gray cat. An exposure based on the white cat will cause the gray cat to appear to be black, while an exposure based only on the gray cat will make the white cat washed out. Light-measuring devices handle this by assuming that the areas measured average a standard value of 18 percent gray, a figure that's been used as a rough standard (not all vendors calibrate their metering for exactly 18 percent gray) for many years.

You could, in many cases, arrive at a reasonable exposure by pointing your 40D at an evenly lit object, such as an actual gray card or the palm of your hand (the backside of the hand is too variable). You'll need to increase the exposure by one stop in the latter case, because the human palm—of any ethnic group—reflects about twice as much light as a gray card. It's more practical though, to use your 40D's system to meter the actual scene, using the options available to you when using one of the Creative Zone modes (P, Tv, Av, M, and A-DEP). (In Basic Zone modes, the metering decisions are handled by the camera's programming.) (See Figure 4.2.)

Figure 4.2

Creative Zone and Basic Zone modes.

Creative Zone modes

Camera User Settings

Figure 4.2

Creative Zone and Basic Zone modes.

Creative Zone modes

Camera User Settings

Basic Zone modes

F/STOPS VERSUS STOPS

In photography parlance, f/stop always means the aperture or lens opening. However, for lack of a current commonly used word for one exposure increment, the term stop is often used. (In the past, EV served this purpose, but Exposure Value and its abbreviation has been inextricably intertwined with its use in describing Exposure Compensation.) In this book, when I say "stop" by itself (no f), I mean one whole unit of exposure, and am not necessarily referring to an actual f/stop or lens aperture. So, adjusting the exposure by "one stop" can mean both changing to the next shutter speed increment (say, from 1/125th second to 1/250th second) or the next aperture (such as f/4 to f/5.6). Similarly, 1/3 stop or 1/2 stop increments can mean either shutter speed or aperture changes, depending on the context. Be forewarned.

In most cases, your camera's light meter will do a good job of calculating the right exposure, especially if you use the exposure tips in the next section. But if you want to double-check, or feel that exposure is especially critical, take the light reading off an object of known reflectance. Photographers sometimes carry around an 18 percent gray card (available from any camera store) and, for critical exposures, actually use that card, placed in the subject area, to measure exposure (or to set a custom white balance if needed).

To meter properly in the Creative Zone, you'll want to choose both the metering method (how light is evaluated) and exposure method (how the appropriate shutter speeds and apertures are chosen). I'll describe both in the following sections.

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Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

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