johnr's chromatic aberration tips

Colour fringe concealment
Hiding the effects of lens chromatic aberration

Modern zoom lenses are miracles of design and engineering, but optically they can be far from perfect. With my camera the performance is worst at wide angles, where fringes of colour round very bright or very dark objects can spoil an otherwise good picture. In this windmill picture, destined to be a 10 x 12 inch print, the dark rail against the white background had a fairly apparent red/cyan border, as the detail enlargement shows.

I've been experimenting with ways to reduce the impact of this effect, which can vary in magnitude and colour. Sometimes the fringes look red/cyan, sometimes green/magenta, but the effect is always progressive away from the centre of the picture and symmetrical about it (that's why I think it's optical and not electronic in origin).

Consider taking a picture of a bright rectangular frame on a dark background.

This is how the colour fringes (exaggerated) might appear. One way of looking at it is that a faint, slightly bigger "ghost" frame, coloured magenta (red+blue), is superimposed on the real white one.

A cunning way to conceal the magenta "ghost" might be to create an enlarged version of the green component of the image, and mix it in at the same level as the magenta one. This still leaves colourless "steps" round the frame, but in practice these are nowhere near as annoying as the coloured fringes.

So much for the theory, now here's how to do it. First you must peer at the original image, greatly magnified, to determine which colour component(s) to resize. In the rectangle example above, an enlarged green component is needed; in the windmill example where the fringe below the white board is red, I could use either enlarged green and blue, or (easier) a reduced red component. You also need to measure (or guess) the width, in pixels, of the fringes.

You need a copy of the appropriate component image. In Corel PHOTO-PAINT (TM), use "Split Channels To ... RGB" from the Image menu. Now, let's assume you've decided to reduce the red component by 6 pixels near the short edge of a 2400 x 1800 pixel image. Since you don't want the centre of the image to move, that means resampling to 2388 x 1791 pixels. Having done this, paste a copy over the original full-colour image (make sure it's central) and set the Merge Mode to Red. Then, by tinkering with the opacity slider (maybe around 25%) you should be able to make the fringes disappear!

Here's the result of processing the windmill image as described, alongside the original detail. It's not perfect but it made a big improvement to the quality of the large print.

Colour fringe concealment Hiding the effects of lens chromatic aberration

Colour fringe concealment
Hiding the effects of lens chromatic aberration