As you probably know, there are no perfect lenses. Even the most sophisticated lens design, developed for costly ($8000 or more!) interchangeable lenses for digital and film SLRs are, at best, compromises of some sort. Lens designers depart from the theoretical "perfect" lens design to add features that photographers demand. Perhaps the lens is intended for low-light photography, so the designer makes a trade-off here or there to allow a wider maximum aperture. Or, the lens must be developed so it is physically shorter, lighter, or can be attached to an SLR camera without interfering with the mirror.
Some lenses are designed so they can better work with ultraviolet illumination for scientific purposes, or optimized for close-up photography. The most challenging lens design of all may be the zoom lens, which with a continuous series of focal lengths, is many lenses in one. There are lots of tricks optical magicians can work with, including non-spherical lens surfaces, special coatings, and combinations of lens elements that move in strange ballets to improve your results. There are even lenses that jostle their elements in response to camera movement to stabilize the image when the shutter speed isn't fast enough.
Because of all the compromises that must be made in building a lens, various types of distortion and aberrations are unavoidable. Photoshop's new Lens Correction filter helps you fix some of them. Here's a description of the most common types of distortions:
• Chromatic Aberration. This is an image defect, often seen as green or purple fringing around the bright edges of an object, caused by a lens failing to focus all colors of a light source at the same point.
• Barrel Distortion. This is a lens defect that causes straight lines at the top or side edges of an image to bow outward into a barrel shape.
• Pincushion Distortion. The opposite of barrel distortion, this defect causes lines at the top and side edges of an image to bend inward, producing an effect that looks like a pincushion.
• Vignetting. If a lens is unable to provide even illumination out to the corners of the image area, the result can be dark corners. This is often found in wide-angle zoom lenses at their widest setting, and when a lens is mounted on an SLR that has a larger sensor size than the lens was designed for. Vignetting can also be produced by using a lens hood that is too small for the field of view, or generated artificially using image-editing techniques.
• Perspective Distortion. This is not, strictly speaking, a defect of a lens but, rather, a result of tilting the camera to take in more of a tall subject, throwing the alignment of the subject and camera focal plane out of whack. I discussed this effect in more detail earlier in the chapter. I've included it here because Photoshop's Lens Correction filter can correct for this kind of distortion.
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