Monday, December 13, 2010

What is chromatic aberation? (CA)


This image is taken by Pentax A 50mm f1.2 at federation square of Melbourne.
Chromatic Aberration

Many a times we come across various Optical terms that seem to fall on deaf ears. Not only they prove to be jargons but also drown us in a state of confusion as it is a human-tendency to start guessing thing which we don’t know.

Chromatic Aberration is one such term that many of you might no be able to explain. A person who uses any form of camera must know this term well as it is closely related to property of a camera-lens.Starting with the basic meaning of Aberration, it is stated that it is the inability of a lens to produce an exact image, particularly at the edges of the photograph. Hence Chromatic Aberration would simply mean the inability of a lens to converge all the wavelengths to a single common point, thus causing a blur. You can also say that Chromatic Aberrations are color deviations of lens systems.This is a common problem in large aperture telephoto lenses that are used to detect astronomical activities and even shoot the closest wild pictures of rare species.In modern times, most of the expensive cameras have special lenses that are protected from such defects. But in most of the normal cameras, the problem of Chromatic Aberration persists.

Every one of us has heard a common term in photography, ‘Purple Fringing’. The extremely small micro lenses of our digital camera that are used to collect more light for each CCD pixel are usually built and fine-tuned in such a way that they can focus the green wavelength perfectly.

The inability to focus the red and blue wavelengths results in purple fringing around the images.Ordinary cameras which have a very small pixel-pitch as in non-DSLR digital cameras have to incorporate a special processing step to remove it.Sometimes extremely sharp digital cameras may face the same problem. The image captured by the lens might have some really small areas. This tiny area is unable to stimulate red, blue and green color pixels and due to absence of some wavelength the captured image has some portion stored with incorrect color.Some of you might be thinking that avoiding colored photography is the simplest solution to this problem. But you are in for a surprise again because Chromatic Aberration affects Black & White Photography also. The phenomenon blurs the image completely. This problem can be overcome by exposing your original image for a longer duration of time to the lens.A very common example to experience is the blooming period for new leaves.

At the time of sunrise, try capturing the image of the filtered sun-rays that pass in between the new leaves and reach the ground. When you see the image, you’ll see the increasing visibility of purple-fringing.Hence, it is a good piece of advice to thoroughly check the lens features for Chromatic Aberration when you go to buy a new camera.