Page 97 - (DK) Advanced Photography Guide

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OPTICAL PROBLEMS

No lens is perfect, and all of them suffer from one or more all optical problems are serious, and they can often be cured
optical problems. Flaws are usually seen when a lens is set either by selecting a lens-correction option in-camera or
to an aperture at either end of the range. You can usually by applying a postproduction fix. Be sure to zoom in on
reduce—or even eliminate—these optical problems by your images, because some problems, such as chromatic
selecting one of the midrange apertures. Fortunately, not aberration, may only be visible at 100% magnification.

VIGNETTING ◀ Vignetting is most obvious
when shooting large areas
The corners of an image are generally darker than
its center when a lens is at the maximum aperture. of light tone, such as soft-
focus background greenery.
This vignetting is usually cured by using a midrange It is not always a problem,
aperture. Vignetting can also be removed though, because you can use
in-camera or in postproduction. However, the it creatively to emphasize
higher the ISO used, the more difficult it is to central subjects.
remove vignetting without losing image quality.

CHROMATIC ABERRATION ◀ When shooting JPEGs,
transverse chromatic
Colored fringing around high-contrast edges in
a photo is known as chromatic aberration, and aberration can be cured
in-camera, but axial cannot.
there are two types: axial and transverse. Axial If you shoot Raw, both
aberrations are seen across a photo taken with a transverse and axial
lens at maximum aperture but disappear as you aberrations have to be
use a smaller aperture. Transverse aberrations are removed later. This photo
seen at the edges, regardless of aperture size. has yet to be repaired.

DISTORTION ◀ Wide-angle lenses
are particularly prone
Some degree of distortion is often visible in
lenses, and it takes one of two forms: barrel and to distortion, usually
toward the edge of the
pincushion. Barrel distortion makes straight lines frame. It can be corrected
bow out toward the edge of the frame, whereas in postproduction,
pincushion makes them bow inward. Zoom albeit with some loss
lenses often show barrel distortion at one end of image resolution.
of the zoom range and pincushion at the other.

LENS TECHNOLOGY TERM DESCRIPTION
Manufacturers are continually Low-dispersion glass This type of glass is specifically designed to minimize
developing new techniques and chromatic aberrations.
materials to improve their lenses,
often trying to minimize the problems Aspherical lens With its complex curved surface, this lens element
described above. These technologies focuses light more evenly and reduces aberrations.
become more important as sensor Applied to the surface of lens elements, coatings help
resolutions increase. The table on the Coatings to reduce the risk of flare.
right explains some of the terms used.
These lens optics move independently of the main lens
Floating elements
elements to improve close focusing.

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