Street Photography Magazine

of your particular lens and you should perform

the adjustments on a RAW image file. These

preconditions usually produce great results

with automatic computer-based or in-camera

correction tools. Lateral errors don’t produce

‘hard’ double images the way axial errors

often do, but it is still virtually impossible to

completely erase edge softness and loss of

contrast after shooting. It is nevertheless

relatively easy to filter out obvious color

fringes.

Double Images and

False Colors

Axial aberrations are anomalies caused by

the different component colors focusing at

different points along the optical axis. This

results in multiple, single-color images

instead of a single, sharp, multi-color image.

Like the sub-images produced by lateral

aberrations, those produced by axial

aberrations tend to appear at high-contrast

edges. However, they neither vary in size nor

depend on the direction of the incident light.

Axial aberrations cause fringing and image

softness across the entire frame and are

particularly obvious at wider apertures (i.e., in

images with shallow depth of field). The

critical factor here is the focus setting rather

than the position of an object within the

frame. Lateral errors increase toward the

edges of the frame.

Depending on the type of subject, it is

possible to reduce the incidence of axial

aberrations by stopping down the lens.

Smaller apertures increase depth of field and

reduce the probability of a single color

channel appearing unsharp in the final image.

The obvious reduction in sharpness

caused by axial aberrations manifests itself in

the form of double images at high-contrast

edges. Axial aberrations can also produce

whole areas of false colors that often tend

toward magenta and are particularly obvious

in brighter image areas. This leads to color

casts in image areas that lie outside the area

of sharp focus.

The spectral color of such areas changes

according to whether the affected image

detail lies in front of or behind the focal

point. Axial aberrations seldom occur at the

focal point itself because its depth of field is

usually sufficient to cover all three color

channels.

Due to their predominant magenta color,

axial aberration artifacts are often known as

‘purple fringing’. However, this is not a

particularly precise term, as other types of

artifacts – such as image noise, blooming and

stray ultraviolet or infrared light – can cause

similar looking effects. Even lateral

aberrations can be confused with axial ones

under certain circumstances.

Computer-based correction of axial

aberrations is only possible to a limited

degree, as no tool exists that can effectively

adjust a single unsharp color channel.

Fringing and color casts can be removed in a

general sense, but this type of correction

always results in an additional reduction in

image sharpness that tests the limits of even

the smartest algorithms.

Other Types of Image Error

In addition to the types of chromatic

aberrations we have mentioned, there are

various other phenomena that can lead to

fringing and color errors, including stray UV

and infrared light. Compact cameras especially

have a high degree of automatic error

Chromatic Aberrations | Basics

Most of today’s image sensors are arranged according to the Bayer

Pattern, which alternates rows of green/blue/green and red/

green/red photoreceptors. In order to be displayed on a monitor

or saved as an image file, the data captured by the

photoreceptors has to be converted to an RGB

format made up of pixels. Each pixel contains red,

green and blue sub-pixels. This conversion process

is known as interpolation and is performed using a

demosaicing algorithm.

Algorithms designed for correcting chromatic

aberrations are much more effective when applied to RAW image

data than they are when used on pre-processed JPEG image files. If

you want or have to shoot in JPEG format, you should use a camera

that has built-in color correction functionality. All Nikon DSLRS since

the D300 automatically correct lateral color aberrations at the edges

of the frame. This feature cannot be configured separately in the

camera’s firmware, but can be adjusted manually

using Nikon’s Capture NXsoftware package.

Camera hardware, too, can influence the creation

of chromatic aberrations. Microlenses positioned

in front of the image sensor ensure that the light

entering the lens is distributed evenly across the

sensor, but also have a reputation for fostering

chromatic aberrations. Even if they have identically sized sensors

and are fitted with the same lens, different camera models often

produce fringing artifacts with quite different characteristics.

In-camera Error Correction

Lateral aberrations

are caused when the

light entering the lens

is split into its

component colors.

The resulting rays,

with their varying

frequencies, hit the

lens at different

points on a plane that

is positioned at right

angles to the optical

axis, causing color

fringing, reduced

contrast and a loss of

image sharpness.