Introducing the Exposure Trio Aperture Shutter Speed and ISO

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Any photograph, whether taken with a film or digital camera, is created by focusing light through a lens onto a light-sensitive recording medium. In a film camera, the film negative serves as that medium; in a digital camera, it's the image sensor, which is an array of light-responsive computer chips.

Between the lens and the sensor are two barriers, known as the aperture and shutter, which together control how much light makes its way to the sensor. The actual design and arrangement of the aperture, shutter, and sensor vary depending on the camera, but Figure 5-2 offers an illustration of the basic concept.

Image sensor

Opened shutter



Image sensor

Opened shutter



Figure 5-2: The aperture size and shutter speed determine how much light strikes the image

The aperture and shutter, along with a third feature known as ISO, determine exposure — what most of us would describe as the picture's overall brightness and contrast. This three-part exposure formula works as follows:

i Aperture (controls amount of light): The aperture is an adjustable hole in a diaphragm set just behind the lens. By changing the size of the aperture, you control the size of the light beam that can enter the camera. Aperture settings are stated as f-stop numbers, or simply f-stops, and are expressed with the letter f followed by a number: f/2, f/5.6, f/16, and so on. The lower the f-stop number, the larger the aperture, as illustrated by Figure 5-3.

The range of possible f-stops depends on your lens and, if you use a zoom lens, on the zoom position (focal length) of the lens. For the kit lens sold with the Rebel XTi/D400, you can select apertures from f/3.5-f/22 when zoomed all the way out to the shortest focal length (18mm). When you zoom in to the maximum focal length (55mm), the aperture range is f/5.6-f/36. (See Chapter 6 for a discussion of focal lengths.)

i Shutter speed (controls duration of light): Set behind the aperture, the shutter works something like, er, the shutters on a window. When you aren't taking pictures, the camera's shutter stays closed, preventing light from striking the image sensor, just as closed window shutters prevent sunlight from entering a room. When you press the shutter button, the shutter opens briefly to allow light that passes through the aperture to hit the image sensor.

Figure 5-3: The smaller the f-stop number, the larger the aperture.

The length of time that the shutter is open is called the shutter speed and is measured in seconds: 1/60 second, 1/250 second, 2 seconds, and so on. Shutter speeds on the Rebel XTi/D400 range from 30 seconds to 1/4000 second when you shoot without flash. Should you want a shutter speed longer than 30 seconds, manual (M) exposure mode also provides a feature called bulb exposure. At this setting, the shutter stays open indefinitely as long as you press the shutter button down.

If you do use a flash, the fastest available shutter speed is 1/200 second; the slowest ranges from 1/60 second to 30 seconds, depending on the exposure mode. See the section "Understanding your camera's approach to flash," later in this chapter, for details.

^ ISO (controls light sensitivity): ISO, which is a digital function rather than a mechanical structure on the camera, enables you to adjust how responsive the image sensor is to light. The term ISO is a holdover from film days, when an international standards organization rated each film stock according to light sensitivity: ISO 200, ISO 400, ISO 800, and so on. Film or digital, a higher ISO rating means greater light sensitivity, which means that less light is needed to produce the image, enabling you to use a smaller aperture, faster shutter speed, or both.

On your camera, you can select ISO settings ranging from 100 to 1600, but only when you shoot in the advanced exposure modes. For the fully automatic modes, you're limited to ISO speeds from 100 to 400, and the camera chooses the setting for you automatically.

Distilled down to its essence, the image-exposure formula is just this simple:

^ Aperture and shutter speed together determine the quantity of light that strikes the image sensor.

^ ISO determines how much the sensor reacts to that light.

The tricky part of the equation is that aperture, shutter speed, and ISO settings affect your pictures in ways that go beyond exposure. You need to be aware of these side effects, explained in the next section, to determine which combination of the three exposure settings will work best for your picture.

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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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