If you already know all about the interrelationships of aperture, shutter speed, and sensitivity, and you routinely calculate equivalent exposures in your head, then you can skip or skim this chapter. But if you're a typical photographer who relies on your camera's automatic exposure system for most of your photographs, then you're probably a little fuzzy about some of the details of what the exposure system is telling you, and perhaps a bit reluctant to step in and override the camera's automation to achieve a desired effect. If your understanding of the elements of exposure is somewhat rusty, this chapter should help.

What Is an Exposure?

In photographic terms, exposure means to allow light (presumably, light that is being focused through a lens) to strike the film or image sensor, thereby recording the image. However, a great deal more happens during exposure than just haphazardly exposing the image sensor to light. You must precisely control the amount of light that reaches the image sensor if you want to get the proper exposure — one that reflects the photographer's vision of the scene with the proper range of tonality and good detail in highlights, midtones, and shadows. You, or your camera's automatic exposure system, adjust the camera's aperture, shutter speed, and ISO (sensitivity) settings, collectively called the elements of exposure ...

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