2. Fast vs. Slow Lenses
Within all lenses, there is something called the aperture. More specifically, the mechanism that makes up the aperture is the diaphragm, a ring of overlapping, thin metal plates that either close down or remain open during exposure. For the sake of clarity, we refer to this mechanism as the aperture.
The aperture controls two things: how much light is let in through the lens and exposed on the sensor (or film), and depth of field, or how much of the shot is in focus. We’ll discuss these two points in the next chapter.
Lenses are often described as either fast or slow, based on their maximum aperture openings. When you are making an exposure in any given light condition, the larger, or more open, the aperture is, the faster the shutter speed needs to be. When you allow a large amount of light to pass through the lens, you need to cut it off more quickly for a proper exposure. Conversely, an aperture that is closed, or stopped down, in that same light condition requires a slower shutter speed to create the exact same exposure (Figures 1.3 and 1.4). Essentially, faster lenses are those with large maximum apertures, and those that cannot open up as much are referred to as slower lenses.
ISO 200 • 1/1000 sec. • f/2.8 • 46mm
ISO 200 • 1/250 sec. • f/5.6 • 46mm
Nikon makes several variations of many of its lenses, and much of the variance is aperture based. For example, there are two different 50mm lenses in the Nikon lineup that come with a built-in focusing motor: the AF-S Nikkor 50mm f/1.4G and the AF-S Nikkor 50mm f/1.8D (plus an older model, the AF Nikkor 50mm f/1.8D). The number following the f-stop is used to identify the maximum aperture opening—the lower the number, the larger the opening.
The AF-S Nikkor 50mm f/1.4G is faster than the AF-S Nikkor 50 f/1.8D. Practically speaking, the AF-S Nikkor 50mm f/1.4G—at its maximum aperture—lets in more light and allows the shooter to use faster shutter speeds than the other allows. This comes in handy when you are shooting in low-light conditions (Figure 1.5). As we’ll discuss later, faster lenses are also valuable on the sports field.
Figure 1.5 Shooting live music requires a fast aperture to freeze the action and handhold longer lenses.
ISO 1600 • 1/320 sec. • f/2.8 • 200mm
These 50mm lenses are non-zoom lenses (also known as prime lenses, covered later in this chapter). Nikon also manufactures many great zoom lenses, and in doing so, introduces another issue—speed of zoom lenses. Some lenses, such as the AF-S Nikkor 24–70mm f/2.8G ED and the AF-S Nikkor 70–200mm f/2.8G ED VR II have a fixed f/2.8 maximum aperture opening. However, other lenses, such as the AF-S Nikkor 18–55mm f/3.5–5.6G VR II and the AF-S Nikkor 70–300mm f/4.5–5.6G IF-ED, have what is commonly known as variable maximum apertures. Simply put, lenses with variable maximum apertures will close down their maximum aperture as they are zoomed in to the scene. For example, when you are shooting with an AF-S DX Nikkor 18–55mm f/3.5–5.6G VR II lens at 18mm—the widest focal length on that lens—the maximum aperture is f/3.5. When zoomed in to 55mm, though, the maximum aperture changes to f/5.6. Along the zoom range, as the focal length increases, the maximum aperture changes to a slower aperture.
These types of lenses are fairly common, and the variable maximum aperture is a result of using smaller-diameter lens elements in the lens, which subsequently keeps costs down. It’s worth noting here that the faster the lens, the more expensive it is. It takes more materials to manufacture faster lenses—more glass, more housing for that glass—since the diameter of the lenses is larger.
Lens speed also refers to how a lens handles depth of field, so keep in mind that the faster the lens, the more one can theoretically throw the background (and foreground for that matter) out of focus. Consider again the 50mm example above. The AF-S Nikkor 50mm f/1.4G, while set at f/1.4, will produce a softer bokeh (the area of the image that goes out of focus when using a faster aperture) than the AF-S Nikkor 50mm f/1.8D (Figure 1.6). It might be a stretch to see the difference between f/1.4 and f/1.8, but it is there. A more extreme example is the difference between the iconic sports lens, the AF-S Nikkor 400mm f/2.8G ED VR used at f/2.8, and the AF-S Nikkor 80-400mm f/4.5–5.6G ED VR used at 400mm with a maximum aperture of f/5.6. There is a considerable difference between the lowest amounts of depth of field each produces when set to maximum aperture.
Figure 1.6 An aperture lower than f/2.8 offers a unique way of isolating portrait subjects from otherwise distracting backgrounds. At f/1.8, there is only a sliver of the baby in focus, while the rest of the foreground and background soften drastically.
ISO 50 • 1/200 sec. • f/1.8 • 50mm