[Optics Column]
• Causes and types of aberrations
In the ideal photographic lens, light from each object point forms a corresponding image point on the image plane. If you look only at light near the optical axis, and consider only a specific wavelength, it is possible for a lens to achieve nearly ideal image formation.
It is virtually impossible, however, with large-aperture lenses to achieve ideal image formation while maintaining sufficient brightness across the entire image field because (1) light rays from each object point do not focus perfectly on the image point when they pass through a spherical lens and (2) different light wavelengths form images at slightly different positions on the image plane.
Aberrations, then, are the differences caused by these factors between ideal image formation and the actual image formed by a real lens. Aberrations are the primary cause of image quality problems in lenses.
There are two main classes of aberrations: those caused by the lens's spherical construction (these are known as Seidel's five aberrations - spherical aberration, coma, astigmatism, field curvature, and distortion, figures 7 to 11), and those caused by wavelength variations (chromatic aberrations, Figure 12).
• Correcting aberrations
Optical design demands that the occurrence of aberrations is minimized and image formation approaches the ideal while simultaneously satisfying the lens specifications, whether wide angle, telephoto, zoom, large aperture, compact, or what have you.
Correcting Seidel's five aberrations is in theory impossible to do with spherical lenses. For this reason we place aspherical lens elements - lenses constructed with aspherical surfaces - at optimal positions in our EF lenses to achieve nearly ideal point-to-point image mapping.
To counter chromatic aberrations, Canon uses special optical elements, such as elements made of fluorite crystal, ultra-low dispersion glass (UD and Super UD) lenses, and Diffractive Optical Element (DO) lenses, that provide excellent correction of chromatic aberration.
Peripheral Brightness
• What is peripheral brightness?
A lens's brightness is determined by the focal length and effective aperture. This, however, is an indication of the lens's brightness along the optical axis; that is, the image brightness in the center of the picture. Conversely, the brightness at the edges of the picture is called the peripheral brightness.
Inevitably the peripheral brightness drops off because of the physical effects of vignetting (a phenomenon where light incident to the edges of the picture is partially blocked by frames and other objects in front of or behind the aperture, Figure 13) and the cosine fourth law of illumination falloff (this law holds that light falloff is proportional to the fourth power of the cosine of the angle between the incident light ray and the optical axis, Figure 14). Appreciable falloff of peripheral brightness results in lower image quality, as the edges of the picture become darker than the center.
• Controlling peripheral brightness falloff
A technique to minimize common peripheral brightness falloff is to make the front lens diameter as large as possible, which amplifies the incident light at the periphery.
The problem with this approach is that simply using a larger lens aperture increases the bulk of the overall lens, making it more difficult to handle, and results in a decline in the peripheral image quality. We design the optics of our EF lenses using the latest design technology to strike the perfect balance between resolution, contrast, peripheral brightness, and physical size of the optics in keeping with the lens specifications.
