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maximized. A good number of Canon’s mainstream lenses incorporate aspherical qualities, including such popular models as the EF 24-70mm f/2.8L and the EF 14mm f/2.8L II USM. Of course, in some cases spherical aberrations can be used to artistic effect even if they do not provide a completely natural or accurate representation of the image being photographed. Take, for example, the image in 4-9, where slightly distorted edges help to pop out the color and dimensionality of the main subject.
CHAPTER
CANON PHOTO WORKSHOP / The Science of Lenses
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DIFFRACTIVE OPTICS Lenses do not form perfect images, and there is always some degree of distortion or aberration introduced by the lens that causes the image to be an imperfect replica of the object. Careful design of the lens system for a particular application ensures that the aberration or distortion is minimized. Telephoto and super-telephoto lenses are especially susceptible to chromatic aberration, which is an optical color defect. Diffractive optics (DO) technology, which was designed to correct these problems in telephoto lenses, uses the principle of diffraction, which is to change the direction of a light wave’s path, to create lenses with a strong diffractive element. They also have optical qualities that help to correct color fringing. If you are not sure how to tell if the diffraction of your lens is good, try examining the straight edges of a subject in a photo you’ve taken. If there is a crisp clear edge without prismatic color fringing, then your lens has good diffractive optics. Figure 4-10 shows how different colors can be refracted through the lens in different ways, resulting in fringing.
4-9 ABOUT THIS PHOTO Taken in Tokyo’s famous Tsukiji fish market, these shellfish almost jump off the page when taken with a fisheye lens, emphasizing their color and texture even more. The areas around the side of the image are clearly distorted by the spherical 15mm lens. Taken with an EOS-1D Mark IIn with an EF 15mm f/2.8 Fisheye lens, ISO 640, 1/80 second, f/4.5. ©Serge Timacheff
4-10 ABOUT THIS FIGURE Prismatic color fringing occurs when lenses fail to diffract different colors precisely, as shown here. Note how the focal plane is where all parts of the lens — from the center to the edges — must accurately point rays of light to produce a crisp image.
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