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Correcting Distortion and Color Fringing is a great tutorial with some practical tips on how to deal with the problems of distortion and color fringing in your digital photographs. Here is the introduction to wet your appetite.
'The effect of optical distortion in a lens is to create curved images of straight objects.� Color fringing, or lateral chromatic aberration, is closely related to distortion, and in fact can be thought of as a chromatic variation of distortion.� Most reversed telephoto wide-angle lenses and zoom lenses suffer from noticeable amounts of both distortion and color fringing.� The reason for this is that these types of lenses tend to be highly asymmetrical, which makes correcting distortion and color fringing especially difficult without resorting to expensive aspheric surfaces and exotic glasses.
Also check out Digital Photography School for more free Digital Camera Tips on All Topics.
Distortion is unique among aberrations in that it does not blur the image.� As a result, it is possible to manipulate a digital image with a simple mathematical formula to eliminate� distortion without significantly harming the image sharpness.� A digital image consists of three color channels, so if you can somehow correct the distortion in each channel to precisely the same degree then you will automatically correct color fringing and distortion simultaneously.
The benefits of this type of image correction are a little bit subtle.� Color fringing is often not readily apparent in many images, and only becomes really obvious in extremely high contrast areas such as the transition from pure white to pure black.� Similarly, distortion is often invisible in natural scenes, and only becomes noticeable in scenes such as architecture where straight lines form crucial elements of the composition.� However, for many photographers the subtle improvements possible by eliminating these aberrations can mean all the difference between an ordinary image and a great one.
Many lenses that have mediocre performance when used on a high quality digital camera can take on a whole new life if they are properly calibrated and corrected using the simple techniques described in this tutorial.� Two such lenses in my personal collection are the Nikkor 15mm f/3.5 AI and Nikkor 24mm f/2 AIS, both of which suffer from large amounts of color fringing that seems to be enhanced when used on a digital SLR camera such as the Nikon D1x.� Both of these lenses, and many others, can become stellar performers after proper image processing.
There are a variety of software tools available that can be used to correct distortion and color fringing, but one of the best and most versatile is the Panorama Tools plugin for Photoshop writen by Helmut Dersch.� Despite its impressive capability, Panorama Tools is free, which certainly makes it difficult to ignore.� The only downside to Panorama Tools is that it can be quite intimidating to a first-time user, but hopefully this tutorial will help ease the pain at least a little.' Read More
Posted by Darren in our Tips category on March 21, 2004