Barrel distortion
In the original shot (top), a wide-angle attachment led to
barrel distortion. Using Pinch filter, to squeeze the image
from all sides, with the Shear filter engaged, has helped
correct the problem.
Anybody whose photography involves taking
accurate pictures of buildings or other prominent
archi tectural, or even some natural, features is
likely at some stage to come up against the
problems associated with image distortion.
Problem: When using an extreme wide-angle
lens, or an accessory lens that pro duces a “fish-eye”
effect, the straight lines of any subject elements—
such as the sides of buildings, the horizon, or room
interiors—appear to be bowed or curved in the
image. With ordinary lenses, this effect (known as
distortion) is usually not obvious, though it
is a feature of many zooms. Don’t confuse this
problem with converging verticals (see p. 208).
Analysis: Distortion is caused by slight changes
in magnification of the image across the picture
frame. Lenses that are not symmetrical in construction
(in other words, the elements on either side of the
aperture are not similar) are particularly prone to
this type of imaging distortion, and the asymmetric
construction of zoom lenses means that they are
highly susceptible.
Solution: Corrections using image-manipulation
software may be possible, but turning curves into
straight lines is more difficult than adjusting straight
lines. Try using the Spherize distortion filter; the
degree of flexibility in Photoshop is not very great,
but some other software packages offer more control
and a better chance of correcting distortion. You
could try applying a succession of filters—first, slight
Perspective Distortion, then Spherize, then slight
Shear—or apply filters to just a part of, rather than
to the entire, image.
Do not place straight lines, such as the horizon
or a junction of two walls, near the frame edges.
Avoid using extreme focal lengths—either the
ultra-wide or the ultra-long end of a zoom range.
Use a focal length setting that offers the least
distortion, usually around the middle of
the range.
Place important straight lines in the middle
of the image—this is where distortion is
usually minimal.

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