Julie Adair King - Digital Photography For Dummies

As your camera's eye, the lens plays a huge part in what types of photos you can capture. It determines not only how much of a scene you can record in one shot, but also the quality of the image and certain other characteristics of a photograph.

When considering lenses, pay attention to the specifications outlined in the next sections.

Interchangeable-lens cameras require specific lens types. If you have a Nikon camera body, for example, the lens must have a Nikon mount. That doesn't mean that you have to stick with the manufacturer's lenses; you can get great lenses from third-party makers such as Tamron and Sigma. Again, just make sure that the lens offers the correct mount for your camera (or that you can make it work with an adapter).

Just because you can put a lens on a camera doesn't ensure that it can take advantage of all camera features, however. Autofocusing may not be possible, for example. Check your camera manual for details on what types of lenses support which camera features.

Focal length, stated in millimeters, refers to the distance from the center of the lens to the image sensor.

Now that I’ve done my due diligence in the technical explanation department, allow me to explain focal length in practical terms:

Focal length determines the lens’s angle of view. The shorter the focal length, the more subject area fits in the frame. Increasing focal length narrows the angle of view and makes your subject appear closer and larger. Figure 1-8 illustrates this fact, showing the same scene captured at four focal lengths. (A lower number indicates a shorter focal length.)Some focal length recommendations: Landscape photography: Look for a wide-angle lens, characterized by a focal length of 35mm or shorter. Nature and sports photography: Assuming you'll be shooting at a fair distance from your subject, you need a telephoto lens, which has a focal length of 70mm or longer. Portrait photography: Aim for a focal length in the range of 70–135mm. At other focal lengths, facial features can be distorted. A wide-angle lens, for example, can make your subjects appear sort of like how they look when you view them through a security peephole in a door. And a very long lens can flatten and widen a face. FIGURE 1-8:The shorter the focal length, the wider the angle of view.

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.

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).

As a general rule, prime lenses equate to better-quality photos because a lens can be engineered to optimal performance at only a single focal length. That said, one of my favorite lenses is the super zoom; it has a monster focal length range — 18 to 400mm. Newer lenses perform better in this regard than those manufactured in the past.

Some fixed-lens cameras offer both optical and digital zoom . Optical zoom is a true zoom lens and produces the best picture quality. Digital zoom is a software feature that crops away the outside of the image and enlarges the remaining area, a process that lowers image quality. In other words, pay attention to the optical zoom specs and don’t be too impressed by the digital zoom range.

The aperture is an adjustable hole through which light must pass to reach the image sensor. Aperture size is stated in f-numbers, more commonly referred to as f-stops. A higher number indicates a narrower aperture size. So f/11, for example, results in a smaller aperture opening than f/8.

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.