BetterPhoto Q&A
Category: New Answers

Carolyn Whiteside

Causes of red outlining in bright areas

Red Outlining
ISO 100; f/9; 1/200; focal length 44mm using a 28-135 Canon IS lens; taken at noon

This has happened in low light situations where bright light hits something light in the pic.

Carolyn Whiteside

Some of my RAW and JPG images have red outlining in the bright light areas. What is the cause? I cropped this area from a photo where the remainder has no problem. Look at the back of the white shirt; his arm; and around pants leg to see the red.

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0 June 13, 2008

Alan N. Marcus

Hi Carolyn,

You have discovered an optical phenomenon called chromatic aberration -another name is purple fringing.

Consider that the job of the camera lens is to bend light rays inward causing them to converge thus forming an image at the film or chip location (focal plane). The problem is; each color bends (refracts) a little differently. Since the subject is comprised of different colors, a blue image is caused to form nearer the lens followed by green then yellow then orange then red. The red image forms more distant from the lens. In other words, if we measure the focal length of each of the separate color images we would discover that the red image has the longer focal length and the blue image has the shortest. Thus the red image will be slightly larger than the blue (more magnification). When we scrutinize an optical image we discover that at the edges reveal a fringing comprised of all colors of the rainbow. Chromatic aberration is always present however it becomes more apparent if the edges of objects are juxtaposed against a mundane background.

A converging lens and diverging lens have opposite chromatic aberrations. Thus in modern lens construction, these two types are combined together in an attempt to cause one to counteract the other. To further assist different types of glass are used, some very dense and some lighter in weight. The finished lens is known as achromatic (without color error). Opticians have never achieved thus this is a daydream.

All camera lenses are made using many elements each with a different power. The idea is to construct by means of a series of compromises to neutralize as many of the aberrations as possible. The price of construction plays a big part. A zoom lens is most difficult to construct because the lens elements must change position to permit the focal length to alter.

If you study optics, you might be the one what goes down in history as the vanquisher of chromatic aberrations.

Alan Marcus (marginal technical gobbledygook)
ammarcus@earthlink.net

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0 June 12, 2008

David Van Camp

In addition to what Alan has said, there are things you can do in software to reduce (if not remove) chromatic aberation:

PaintShopPro (PSP) ver 8 and up (and possibly earlier) has a specific feature for this that works fairly decently.

PhotoShopElements (PSE) 4 (my version) unfortunately does not... don't know about later versions.

Don't know about CS. I know there are other products that do, but cannot comment.

I bought PSP v9 on eBay for ~$25, it's a good complement to PSE ... I often tend to use both for editing a single image, as PSP has a number of features that are either missing from, or don't work as well as in PSE. Noise reduction of all kinds works better in PSP, as does perspective correction too, and some 3rd party plug-ins work better as well.

dvc

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0 June 12, 2008


	CAfix W.

Adding to David's remarks:

in Camera RAW 4.4.1 – which opens when you open a RAW file in CS3 – you can fix different kinds of chromatic aberation under 'lens corrections'.