As you've learned, the shutter sits just in front of the image sensor and opens and closes to expose the sensor to light. The shutter speed that you (or the camera) chooses determines how long the shutter will stay open.
Shutter speeds are measured in seconds, and a longer shutter speed exposes the sensor to light for a longer period of time than a shorter shutter speed. The T1i has a shutter speed range of 1/4,000th of a second on the fast end to 30 seconds on the slow end. The T1i also provides a Bulb mode, which lets you keep the shutter open indefinitely.
To a photographer, a stop is a measure of light. More specifically, a stop represents a doubling of light. So, any time there's a doubling of light—whether we're talking about the light in a scene or the light hitting your sensor—we say that the amount of light has increased by one stop. Conversely, a halving of light means that the light has decreased by one stop. We will be using this term extensively throughout the rest of the book. Experienced photographers are often able to recognize one or two stops of light by eye. While that can take years of practice, it's not a necessary skill to use your camera well. However, you will need to understand the term and how it applies to the different exposure settings.
In the Full Auto shooting that you've been doing, the camera has been calculating an appropriate shutter speed for you automatically, and you can see its decision in the camera viewfinder status display.
Here, the camera is indicating that it has chosen a shutter speed of 1/125th of a second.
On the status display shown on the back of the camera, shutter speed is the number in the upper-left corner.
Shutter speed is also shown on the rear LCD screen. In later chapters, you'll learn how to control shutter speed manually.
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Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.