Full-time manual focusing comes in very handy, for example, with macro shots and when using extension tubes. An advantage of Canon lenses is the ability to use autofocus, and then tweak focus manually using the lens's focusing ring without switching out of autofocus mode or changing the switch on the lens from the AF (Autofocus) to MF setting. Macro lenses, which are covered in detail later in this chapter, enable close-up focusing with subject magnification of one-half to life-size and a maximum aperture of f/25. The amount of handholding latitude varies by lens and the photographer's ability to hold the lens steady, but you can generally count on one to three more f-stops of stability with an IS lens than with a non-IS lens. IS lenses allow you to handhold the camera at light levels that normally require a tripod. Lenses labeled as IS lenses offer image stabilization, which is detailed later in this chapter. Moving parts, including the focusing ring and switches, are also designed to keep out environmental contaminants. L-series lenses have rubber seals at the switch panels, exterior seams, drop-in filter compartments, and lens mounts to make them both dust and water resistant. The L-series EF tele-photo lenses stand up well to inclement weather and heavy use. For any photographer who shoots in inclement weather, having a lens with adequate weather sealing is critical. Lenses with aspherical elements are designated as AL. These types of optics help correct distortion in ultra-wide- angle lenses as well.
An aspherical element uses a varying curved surface to ensure that the entire image plane appears focused. Spherical aberration happens when wide-angle and fast normal lenses cannot resolve into a sharp point of focus light rays coming into the lens from the center with those coming from the edge. This technology is designed to help counteract blurred images that happen as a result of spherical aberration. Lenses with UD or Fluorite elements are designated as CaF2, UD, and/or S-UD. On the other hand, Fluorite elements, which are used in super-tele-photo L-series lenses, reduce chromatic aberration. UD elements are used, for example, in the EF 70-200mm f/2.8L IS USM and EF 300mm f/4L IS USM lenses. This glass also provides improved contrast and sharpness. Ultralow Dispersion (UD) glass elements help minimize color fringing or chromatic aberration. L-series lenses include one or more of the following technologies and features: The distinguishing characteristics of L-series lenses, in addition to their sobering price tags, are a combination of technologies that provide outstanding optical performance. Canon's L-series lenses feature a distinctive red ring on the outer barrel, or in the case of telephoto and super-telephoto lenses, are distinguished by Canon's well-known white barrel.
A second design, the microultrasonic motor, provides the advantages of this technology in the less expensive EF lenses. This design is offered in the large-aperture and super- telephoto lenses. In addition, lenses with a ring-type ultrasonic motor offer full-time manual focusing without the need to first switch the lens to manual focus. USM lenses use electronic vibrations created by piezoelectric ceramic elements to provide quick and quiet focusing action with near instantaneous starts and stops. The motor is powered by the camera however, because the lens has its own focusing motor, you get exceptionally fast focus. When you see USM, it indicates the lens features a built-in ultrasonic motor with a very quiet focusing mechanism. Because EF-S lenses have a shorter back focus than EF lenses, the EF-S lenses won't be compatible should you eventually buy a full-frame EOS camera. These lenses tend to be more affordable with good optical resolution. The EF-S lens mount is designed specifically for cropped-image-sensor cameras such as the XSi/450D. In addition, if you use lens extenders, the exposure compensation is automatically calculated. The EF system does a self-test using a built-in microcomputer so that you're alerted of possible malfunctions of the lens via the camera's LCD display. The EF mount is fully electronic and resists abrasion, shock, and play, and needs no lubrication like other lens mounts. The EF lens mount provides not only quick mounting and removal of lenses, but it also provides the communication channel between the lens and the camera body. This designation identifies the type of mount that the lens has and the camera accepts. Canon uses several designations and lens construction technologies that are helpful to know as you consider lenses to add to your camera system.
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