Lens Distortion

Lens Distortion

When dealing with any kind of lenses you eventually have to deal with optical distortion.
The lenses you attach to your camera are a complex system of optics with
many different lens elements that are used to give a certain field of view. Without getting
into the technical details of how a lens works, just know that the lens elements
bend or refract the light manipulating it to magnify it (in the case of a telephoto) or
making it look farther away (in the case of wide-angle lenses). With all of this light,
bending going on, lenses can distort the image-making it look different than the original
subject.



Each type of lens has its own different kind of distortion. For example, wide-angle
lenses suffer from what is known as barrel distortion. Barrel distortion is characterized
by the image being sort of rounded out near the edges of the frame. You can see
in the following figure that the parallel lines at the edge of the image bulge outward.

Telephoto lenses suffer from the opposite problem, which is referred to as pincusion
distortion. This type of distortion is characterized by the edges of the frame appearing
to be pinched in toward the center, as you can see in the figure with the parallel
lines at the edge of the frame bending toward the center.

Both types of distortion are more noticeable near the extreme edges of the frame.
Almost all lenses manufactured today are rectilinear, which means that there are
additional lens elements built into the lens that rectify, or fix, the curved lines. In
wide-angle lenses these elements are sometimes referred to as aspherical. Although
these lens elements are designed to counteract the curvature, no lens design is perfect,
so there will always be some lens distortion, although it is usually minimal.

Fisheye lenses are lenses that have not been corrected for any type of distortion and
the images appear curved or spherical.

Most lens distortion is only noticeable when photographing subjects that have a lot
of straight lines, especially near the edges of the frame, so lens distortion is not
always a problem. However for those times when you may be photographing images
with a lot of straight lines and distortion may be noticeable, some image-editing programs
such as Nikon Capture NX and Photoshop CS3 have tools that allow you to correct
for the distortion.