Modern lenses are complex creations with specialized coatings on the glass to optimize color quality and image sharpness. Figure 3.1 shows a cross-section illustration of a zoom lens. As you can see, multiple lens elements make up a modern lens.
Figure 3.1. As you can see from this illustration, a modern lens is a complex piece of equipment with many glass elements. Image courtesy Canon USA.
These lens elements direct the light coming into the lens to create an image on the sensor in your camera. Early lenses, and many of the less expensive lenses available today, have no coatings on the lens elements, which causes the light to scatter as it travels through the lens to the sensor, resulting in images that look soft or have color problems called chromatic aberration. In the past, many pros avoided zoom lenses in favor of fixed-length, or prime, lenses to avoid these issues. In recent years, though, the quality of zoom lenses has improved greatly to the point where many of the better zooms on the market now rival the prime lenses they compete against.
The other major component of a lens is the diaphragm, which is the iris-type opening at the camera end of a lens. By changing the size of the opening in the diaphragm, you control how much light enters the lens, and therefore how much depth of field, or area of focus, is captured. The size of the opening is the aperture of the lens.
When you start to examine the variety of lenses available on the market, you find two things very quickly. First, there's a wide range of focal lengths, or lens sizes, available for most dSLR cameras. Second, you could easily mortgage your house to purchase some of these lenses. Fast lensesthose with large maximum apertures of f/2.8 or f/4and telephoto lenses can be extremely expensive and heavy to boot. As an example, the Canon EF 500 IS f4L lens, a very popular option with wildlife photographers (Figure 3.2 was shot with it), weighs in at a healthy 8.5 pounds, and costs a wallet-draining $5,500.
Figure 3.2. The Canon EF 500 IS f4L is a popular choice among wildlife shooters, but it's a heavy and expensive lens, partially because of the faster aperture.