Canon Lens Terminology
Canon uses several designations and lens construction technologies that are helpful to know as you consider lenses to add to your camera system.
♦ EF/EF-S lens mount. This designation identifies the type of mount that the lens has and the camera accepts. 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. The EF mount is fully electronic and resists abrasion, shock, and play, and needs no lubrication like other lens mounts. 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. In addition, if you use lens extenders, the exposure compensation is automatically calculated. The EF-S lens mount is designed specifically for cropped-image-sensor cameras such as the XSi/450D. These lenses tend to be more affordable with good optical resolution. 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.
♦ USM. When you see USM, it indicates the lens features a built-in ultrasonic motor with a very quiet focusing mechanism. The motor is powered by the camera; however, because the lens has its own focusing motor, you get exceptionally fast focus. USM lenses use electronic vibrations created by piezoelectric ceramic elements to provide quick and quiet focusing action with near instantaneous starts and stops.
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. This design is offered in the large-aperture and super-telephoto lenses. A second design, the microultrasonic motor, provides the advantages of this technology in the less expensive EF lenses.
♦ L-series lenses. 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. The distinguishing characteristics of L-series lenses, in addition to their sobering price tags, are a combination of technologies that provide outstanding optical performance. L-series lenses include one or more of the following technologies and features:
• UD/Fluorite elements.
Ultralow Dispersion (UD) glass elements help minimize color fringing or chromatic aberration. This glass also provides improved contrast and sharpness. UD elements are used, for example, in the EF 70-200mm f/2.8L IS USM and EF 300mm f/4L IS USM lenses. On the other hand, Fluorite elements, which are used in super-tele-photo L-series lenses, reduce chromatic aberration. Lenses with UD or Fluorite elements are designated as CaF2, UD, and/or S-UD. 2
• Aspherical elements. This technology is designed to help counteract blurred images that happen as a result of spherical aberration. 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. An aspherical element uses a varying curved surface to ensure that the entire image plane appears focused. These types of optics help correct distortion in ultra-wide-angle lenses as well. Lenses with aspherical elements are designated as AL.
• Dust, water-resistant construction. For any photographer who shoots in inclement weather, having a lens with adequate weather sealing is critical. The L-series EF tele-photo lenses stand up well to inclement weather and heavy use. 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. Moving parts, including the focusing ring and switches, are also designed to keep out environmental contaminants.
♦ Image stabilization. Lenses labeled as IS lenses offer image stabilization, which is detailed later in this chapter. IS lenses allow you to handhold the camera at light levels that normally require a tripod. 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.
♦ Macro. 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.
♦ Full-time manual focusing. 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. Full-time manual focusing comes in very handy, for example, with macro shots and when using extension tubes.
♦ Inner and rear focusing. Lenses' focusing groups can be located in front of or behind the lens diaphragm, both of which allow for compact optical systems with fast AF. Lenses with rear optical focusing, such as the EF 85mm f/1.8 USM, focus faster than lenses that move their entire optical system, such as the EF 85mm f/1.2L II USM.
♦ Floating system. Canon lenses use a floating system that dynamically varies the gap between key lens elements based on the focusing distance. As a result, optical aberrations are reduced or suppressed through the entire focusing range. In comparison, optical aberrations in non-floating system lenses are corrected only at commonly used focusing distances. At other focusing distances, particularly at close focusing distances, the aberrations appear and reduce image quality.
♦ AF Stop. The AF Stop button, offered on several EF IS supertelephoto lenses, allows you to temporarily suspend autofocusing of the lens. For example, if the lens is focusing and an obstruction comes between the lens and the subject, you can press the AF Stop button to stop focusing to prevent the focusing from being thrown off. Once the obstruction passes by the subject, the focus remains on the subject, provided that the subject hasn't moved, so that you can resume shooting.
Diffractive optics. Diffractive optics (DO) bonds diffractive coatings to the surfaces of two or more lens elements. The elements are then combined to form a single multilayer DO element designed to cancel chromatic aberrations at various wavelengths when combined with conventional glass optics. Diffractive optics result in smaller and shorter telephoto lenses without compromising image quality. For example, the EF 70-300mm f/4.5-5.6 DO IS USM lens is 28 percent shorter than the EF 70-300mm f/4.5-5.6 IS USM lens.
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