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Manipulating Exposure Controls

The lens admits light. The diaphragm and shutter control how much. In a conventional camera, or in a digital camera that's based on a conventional camera body, these are mechanical devices. The diaphragm controls the brightness, or intensity, of the light. The shutter controls the duration of the light.

Aperture

Aperture means opening. It refers to the opening or hole left by the diaphragm that allows light to pass through the lens. Essentially, the diaphragm is a set of metal plates or leaves that open and close to make a larger or smaller hole. The diaphragm is mounted in between the glass elements of a lens. When it's open all the way, the lens admits as much light as possible. When it's a tiny hole, obviously, it doesn't admit much light at all. The diaphragm openings, or apertures, are called f/ numbers or f/stops. They're assigned numbers according to the diameter of the diaphragm opening, expressed as a ratio of the focal length of the lens.

Confusing? Maybe an example will help make this clearer. Let's say you have a 50mm lens. That means the focal length, or the distance from the optical center of the lens to the film plane, is 50mm (see Figure 3.10). The widest aperture possible on that particular lens is 25mm. That gives you a ratio of 1:2, usually expressed as f/2. A different 50mm lens can have a bigger piece of glass and a wider aperture. It might be, for example an f/1.8 lens.

Figure 3.10Figure 3.10 Because the focal length is twice the maximum aperture, this is an f/2 lens.

Diaphragms would be useless if they were always wide open, so the lens has settings for smaller apertures. When you change the f/stop, the blades of the diaphragm move apart or together to produce a larger or smaller hole. To make matters more confusing, the smallest numbers are the largest openings, and the largest numbers are the smallest openings. It works this way because the f/stops represent fractions of the focal length of the lens. Just as 1/4 is twice as large as 1/8, f/4 is twice as wide an aperture as f/8.

Digital cameras use an automatic exposure system to measure the light and adjust the shutter and aperture, turning on an internal flash if necessary. On many digital models, you can set the auto exposure system to choose the best overall exposure or to give priority to either the aperture or shutter speed by pressing the Mode button, as shown in Figure 3.11. On many cameras, you can also override the automatic exposure system and choose your own settings. You can compensate for back-lit subjects or use a faster shutter to stop motion.

Figure 3.11Figure 3.11 Press the Mode button to cycle through settings. (This is also an example of a macro lens photo.)

Nevertheless, aperture is a consideration for the digital photographer because it affects several other camera functions, including depth of field. When the lens is set at a smaller aperture (higher number f/stop), the light rays that enter it are doing so through the flattest, optically best part of the lens. Therefore, there is less fuzziness (smaller circles of confusion) and better depth of field than when the entire lens is admitting light. To increase the depth of field, use the smallest possible lens aperture. This may require a very long exposure, or adding more (artificial) light to the scene.

Shutter Issues

In a conventional camera, you must deal with two different mechanisms for controlling the amount of light that reaches the film. The aperture is the first variable. The shutter is the second variable. The shutter opens for a fraction of a second to allow light to reach the film.

Exposure is, therefore, a function of the intensity of the light and the time the light is allowed to strike the sensitive surface. When there's more light, less time is needed. When there's less light, more time is needed. The relationship between these two variables can be expressed as an equation:

Exposure (E) is the product of (=) Intensity (I) multiplied by (x) Time (T).

This equation, E = I x T, is called the Reciprocity Law. I and T are reciprocals, which means if you increase one and decrease the other by the same factor, the result is the same.

Of course, you must use some sort of measuring system to determine how much light is falling on the subject. Many conventional cameras and all digital cameras are equipped with light meters that measure the amount of light reflected from the subject to the camera. A microchip inside the camera takes the light measurement and computes a suitable combination of aperture and shutter speed to expose the film correctly. If your picture requires a certain shutter speed, it calculates the correct aperture. If you want to use a very small aperture to gain as much depth of field as possible, the camera sets itself to the appropriate shutter speed.

In a camera with a fixed aperture lens, the shutter speed varies according to how much light is available. Digital cameras use an electronic shutter instead of the mechanical ones found in conventional cameras. Effective electronic shutter speed can be anywhere from 1/2 to 1/4,000th of a second.

TIP

When you must use a fast shutter speed to capture something in motion and your camera doesn't enable you to set the shutter speed, you can trick it by setting the aperture to its widest opening, and by making sure the subject is brightly lit. This forces the electronic shutter to open and close very quickly, so as not to overexpose the picture.

White Balance

Digital cameras have a capability not possessed by the conventional camera. In addition to measuring how much light falls on the subject, most digital cameras also measure the color temperature of the light and adjust accordingly so that any white objects in the picture will be white, rather than yellow, green, or some other color. Color temperature is measured on the Kelvin scale, and can best be explained if you think of a piece of metal being heated. When it's cold, it is black. As it is heated, it eventually glows a dull red, and then a bright cherry red, and then orange, yellow, on up to a brilliant blue-white. Table 3.1 shows the color temperature of some typical light sources.

Table 3.1 Color Temperature of Typical Light Sources

Light Source

Approximate Color Temperature (in Degrees Kelvin)

Candle flame

1,930

Dawn sunlight

2,000

Household light bulb

2,760–2,960

"Warm-white" fluorescent lamp

3,000

Photoflood bulb

3,400

"Daylight" fluorescent lamp

4,500

Sunlight at noon

5,400

Average daylight (sun and sky combined)

6,500

Blue sky

12,00–18,000


As you can see, daylight is a lot bluer (higher on the Kelvin scale) than indoor light. That's why pictures taken outdoors may have a bluish cast, or pictures taken inside may seen to have a warm, orange tone. Most, but not all, digital cameras, attempt to find something white in the scene and compare it to their internal reference on what white should look like. They then adjust their levels to match. If the camera can't compensate for the quality of light, you can adjust it when you upload your pictures to the computer. Sometimes, though, the effect of unadjusted whites can be very helpful.

In the picture in Figure 3.12, the cat was lit only by daylight coming in the window. It was, as you can see, a snowy, overcast day. Even though the sky was gray rather than blue, the color temperature was close to that of blue sky. The cold tones made the black and white cat even more black and white. The softly diffused light illuminates him evenly, avoiding highlights and shadows, and adding a slightly hazy, romantic quality to the portrait.

Figure 3.12Figure 3.12 The light in this photo creates a pensive pussycat.

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