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       Note, however, that the focal lengths stated in this book are so-called 35mm equivalent focal lengths. Here’s the deal: When you put a standard lens on most dSLR cameras, including your 80D, the available frame area is reduced, as if you took a picture on a camera that uses 35mm film negatives and then cropped it.

This so-called crop factor varies depending on the camera, which is why the photo industry adopted the 35mm-equivalent measuring stick as a standard. With the 80D, the crop factor is roughly 1.6. So the 18–55mm kit lens, for example, captures the approximate area you would get from a 29–88mm lens on a 35mm film camera. (Multiply the crop factor by the lens focal length to get the actual angle of view.) In Figure 1-2, the red line indicates the image frame area that results from the 1.6 crop factor.

      When shopping for a lens, remember this crop factor to make sure that you get the focal length designed for the type of pictures you want to take.

      ❯❯ Prime versus zoom lenses: A prime lens is a single focal-length lens. With a zoom lens, you get a range of focal lengths in one unit. For example, the kit lens we feature in this book has a focal-length range of 18–55mm.

      Why select a lens that offers a single focal length when a zoom lens offers a range of focal lengths? In a word, quality. Because of some lens science we won’t bore you with, you typically see some reduction in picture quality at certain points in the range of a zoom lens. On the flip side, a zoom lens is more convenient than carting around a bag of prime lenses, and many zoom lenses today offer very good image quality.

      ❯❯ Aperture range: The aperture is an adjustable diaphragm in a lens. By adjusting the aperture size, you can control the amount of light that enters through the lens and strikes the image sensor, thereby controlling exposure. The aperture setting also affects depth of field: A wide-open aperture produces a short depth of field, so the subject is sharply focused but distant objects appear blurry; a narrow aperture produces a long depth of field so that both the subject and distant objects appear sharp.

      Chapters 7 and 8 cover these issues in detail. For the purposes of lens shopping, you need to know just a few things.

      ● Every lens has a specific range of aperture settings. Obviously, the larger that range, the more control you have over exposure and depth of field.

      ● The larger the maximum aperture, the “faster” the lens. Aperture settings are stated in f-stops, with a lower number meaning a larger aperture. For example, a setting of f/2 results in a more open aperture than f/4. And if you have one lens with a maximum aperture of f/2 and another with a maximum aperture of f/4, the f/2 lens is said to be faster because you can open the aperture wider, thereby allowing more light into the camera and permitting the image to be captured in less time. This not only benefits you in low-light situations but also when photographing action, which requires a fast shutter speed (short exposure time). So, all other things being equal, a faster lens is better.

      ● With some zoom lenses, the maximum and minimum aperture change as you zoom the lens. For example, when you zoom to a telephoto focal length, you usually can't open the aperture as much as you can at a wide-angle setting. You can buy lenses that maintain the same maximum and minimum aperture throughout the whole zoom lens, but you pay more for this feature.