1. Fast vs. Slow Lenses
Within all Canon 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 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 1.1 and 1.2). Essentially, faster lenses are those with large maximum apertures, and those that cannot open up as much are referred to as slower lenses.
Figure 1.1 At f/2.8, the lens’s aperture is considered fast since it allows much more light in at one time than a slower aperture value, such as f/5.6 (see Figure 1.2).
ISO 200 • 1/1000 sec. • f/2.8 • 46mm
Figure 1.2 An aperture value of f/5.6 is two stops slower than f/2.8, meaning the shutter speed used in this shot is two stops slower than the speed used in Figure 1.1.
ISO 200 • 1/250 sec. • f/5.6 • 46mm
Canon makes several variations on many of their lenses, and much of the variance between them is aperture based. For example, there are three different 50mm lenses in the Canon lineup: the flagship 50mm f/1.2L, the 50mm f/1.4, and the 50mm f/1.8 (also known as the “nifty fifty”). The number following the f-stop is used to identify the maximum aperture opening—the lower the number, the larger the opening.
The 50mm f/1.2L is the fastest of the 50s, and the 50 f/1.8 is the slowest. Practically speaking, the 50mm f/1.2L—at its maximum aperture—lets in more light and allows the shooter to use faster shutter speeds than the other two 50s allow. This comes in handy when shooting in low-light conditions (Figure 1.3). As we’ll discuss later, faster lenses are also valuable on the sports field.
Figure 1.3 Shooting live music requires a fast aperture to freeze the action and handhold longer lenses, even if the shot is a tight headshot of a keyboard player.
ISO 2000 • 1/320 sec. • f/2.8 • 93mm
These three lenses are non-zoom lenses (also known as prime lenses, covered later in this chapter). Canon also manufactures many great zoom lenses, and in doing so, introduces another issue—lens speed. Some lenses, such as the 24–70mm L and the 70–200mm L have a fixed f/2.8 maximum aperture opening. However, other lenses, such as the 18–55mm f/3.5–5.6 and the 70–300mm f/4–5.6, 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 shooting with an 18–55mm f/3.5–5.6 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 are larger. Oftentimes, the fastest lenses are reserved for the L series. These lenses are made with premium materials, glass being just one of them (more on this subject later in this chapter as well).
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 50mm f/1.2L, while set at f/1.2, will produce a softer bokeh (the area of the image that goes out of focus when using a faster aperture) than the 50mm f/1.4 and f/1.8 (Figure 1.4). It might be a stretch to see the difference between f/1.2 and f/1.4, but it is there. A more extreme example is the difference between the iconic sports lens, the 400mm f/2.8L, and the nimble-yet-razor-sharp 400mm f/5.6L. There is a considerable difference between the lowest amounts of depth of field each produces when set to maximum aperture.
Figure 1.4 An aperture lower than f/2.8 offers a unique way of isolating portrait subjects from otherwise distracting backgrounds. At f/1.2, there is only a sliver of carpenter Jason Duby in focus, while the rest of the foreground and background soften drastically.
ISO 50 • 1/200 sec. • f/1.2 • 50mm