Focal length affects depth of field. As focal length goes up, depth of field — the distance over which focus appears sharp — goes down. As an example, compare the backgrounds in Figure 1-8. Notice how much blurrier the trunk of the palm tree behind the sculpture appears in the 170mm image than in the versions shot at the shorter focal lengths.
The angle of view produced by any focal length depends on the camera’s crop factor. For reasons too wonky to get into, the photo industry still measures lens focal lengths using the traditional 35mm film negative as a standard. That means that you get the stated focal length — and resulting angle of view — only on a camera that has a full-frame sensor (one that's the same size as a 35mm film negative). With a camera that has a smaller sensor, the angle of view is reduced because the sensor is no longer large enough to capture the entire area that the lens can see. The resulting picture is what you would get if you took a picture with a full-frame camera and then cropped the picture. The measurement of how much frame area you lose is known as the crop factor.Because sensor sizes vary, the crop factor depends on the camera model. Most dSLR and mirrorless image sensors have a crop factor ranging from 1.5 to 2. Figure 1-9 illustrates the image area at these crop factors when compared to the full-frame view.To figure out what angle of view a lens will provide, multiply the lens focal length by the camera’s crop factor, which should be stated in the camera specs. For example, if the camera has a crop factor of 1.5, a 50mm lens gives you the same angle of view as a 75mm lens on a full-frame digital or 35mm-film camera.
FIGURE 1-9: The white, red, and blue boxes indicate the angle of view you get with cameras that have crop factors of 1.5, 1.6, and 2.0, respectively.
In most cases, focal length is printed on the lens, but for some models, you may need to check the user manual or lens spec sheet. Often, the manufacturer gives both the actual focal length of the lens (that's the measurement mentioned in the opening to this section) as well as the 35mm equivalent.
Prime versus zoom lenses
A prime lens offers a single focal length; a zoom lens, a range of focal lengths. For example, a lens might zoom from 18 to 55mm.
In camera or lens advertisements, the zoom range is sometimes described in terms of an “x” factor, as in a 3x zoom. Here, the x means times, with the value indicating the difference between the shortest and longest focal length of the lens. So an 18–55mm lens boasts a 3x zoom, for example (18 × 3 = 54).
Lens aperture range
Changing the aperture size is one way to manipulate exposure. But the f-stop setting also contributes to depth of field, or the distance over which focus appears sharp. The smaller the aperture, the greater the depth of field, as illustrated in Figure 1-10. The background is much sharper in the left image, taken with an f-stop of f/22, compared with the right image, shot at f/6.3.
If you're keeping track, you now know that the lens gives you two points of control over depth of field: the focal length and the aperture setting. In Figure 1-10, I used the same focal length for each shot, so the aperture setting is the sole reason for the shift in depth of field.
FIGURE 1-10: For the left image, I set the aperture to f/22; for the right image, f/6.3.
And why, you're probably wondering, is the exposure of both images in Figure 1-10 the same, given what I just said about the f-stop affecting image brightness? This is why: To compensate for opening the aperture to f/6.3, I reduced the exposure time by changing the shutter speed from 1/320 of a second to 1/4000 of a second, so the light was able to strike the image sensor for a shorter period. I kept the ISO setting (light sensitivity) at ISO 400 for both photos.
You can explore f-stops, exposure, and depth of field further in Part 2 of the book. For the purpose of comparing lenses, you need just a few more bits of aperture information:
Every lens has a specific range of aperture settings. Obviously, the larger that range, the more control you have as a photographer.
The larger the maximum aperture, the “faster” the lens. Again, the more open the aperture becomes, the less time is needed to expose the image. So if one lens can open to a maximum setting of f/4 and another lens has a maximum aperture of f/2, the f/2 version is said to be faster. A fast lens is especially beneficial when photographing action, because a moving subject blurs at long exposure times. But it also helps when you shoot in dim lighting, because you can get the shot at a lower ISO setting, reducing the chances of image noise. That very low f-stop also enables you to produce images that have a very shallow depth of field when your creative vision demands it.
On a zoom lens, the aperture range may change as you zoom in or out. For example, on an 18–140mm lens, you may be able to open the aperture to f/2 when the lens is at the 18mm position but only to f/5.6 at 140mm. You can buy zoom lenses that maintain the same minimum and maximum apertures throughout the zoom range, but be prepared to part with more money than for a lens that doesn't offer this feature.
Depth of field at any aperture varies depending on the size of the image sensor and lens. Cameras with small sensors and lenses produce a much greater depth of field at any f-stop than cameras with larger sensors and lenses. The result is that it can be difficult to achieve much background blurring even if you open the aperture all the way. That's an important consideration if you're interested in the type of photography that benefits from a short depth of field, such as portraiture. On the other hand, if you're a landscape photographer, you may love the extended depth of field those smaller sensors and lenses produce.
Minimum focusing distance
This number is especially important if you enjoy shooting close-ups. The shorter the minimum focusing distance, the closer you can get to your subject, enabling you to fill the frame with small details.