A camera, shown in FIGURE 35.3, “takes a picture” by using a lens to focus a real, inverted image on a light-sensitive detector. The camera lens is always a combination of two or more individual lenses. Real camera lenses are quite complex; a professional-quality lens may have as many as eight individual lenses. Fortunately, the simple two-lens model of FIGURE 35.4 illustrates the main ideas. This combination of converging and diverging lenses corrects some of the defects inherent in a single lens, as we’ll discuss later in the chapter.

FIGURE 35.3 A camera.

FIGURE 35.4 A simple camera lens is a combination lens.

A zoom lens changes the effective focal length by changing the spacing between the converging and diverging lenses. This is what’s happening when the lens barrel on a camera moves in and out as you use the zoom. For objects more than about 10 focal lengths from the lens, which would typically be no more than about 50 cm, the condition $s\gg f_{\text{eff}}$ (and thus $1\text{/}s\ll 1\text{/}{f}_{\text{eff}})$ leads to the approximation $s\prime \approx f_{\text{eff}}$. In other words, objects more than about 10 focal lengths away are essentially at infinity, so parallel rays are focused one focal length behind the lens.

For such an object, the lateral magnification of the image is

Equation 35.3 tells us that the size of the image is directly proportional to the effective focal length of the lens. The effective focal length is easily changed by varying the distance between the individual lenses, and this is exactly how a zoom lens works. A lens that can be varied from $f_{\text{eff min}}=6\text{mm}$ to $f_{\text{eff max}}=18\text{mm}$ gives magnifications spanning a factor of 3, and that is why it is specified as a $3\times$ zoom lens. This is typical of the lens on a point-and-shoot camera.