Sharpness and Chromatic Aberration

Most of us want at least some of a photo to be nice and sharp. Even those taken with a shallow depth of field usually need a point of sharp focus. But, sometimes no matter how hard we try, the picture just doesn’t look as sharp as it should. Some of the time, this lack of sharpness is down to noise reduction, camera shake or poor focusing. Other times, it’s the lens which lets us down – especially away from the centre of the image.

Lenses bend light, focusing rays onto the film or sensor. Different frequencies (colours) bend at different amounts, so a point of white light can sometimes be split into a series of colours, like with a prism. Lens manufacturers design their products to keep this effect to a minimum, but it’s almost impossible to eliminate completely. If you look closely enough, you’ll see it in many photos.

Look at your images (especially towards the edges) and find a contrasting boundary – maybe a tree branch or a window frame or something going from light to dark. If you see thin coloured bands along the edges—especially red/cyan and/or blue/amber edges—then you’ve found the lens’s weak spot.

All is not lost. This problem can be very easy to overcome because digital cameras and computers record images as three separate red, green, and blue images. This makes it relatively easy for software to correct these chromatic aberrations.

Software like Adobe Camera Raw (when used with Creative Suite) and Photoshop’s Lens Correction filter are able to ‘tune-out’ chromatic aberrations almost completely. They provide automatic aberration removal, and fine-tuning controls which can be tweaked until these coloured fringes almost vanish.

But, as well as removing unsightly coloured fringes, another major benefit of using these controls is in the fact that it is effectively stretching the different colour channels of your image by tiny amounts, so they line up perfectly, making your whole image sharper as a result.

[UPDATE, March 2012: Adobe’s Lens Correction function has had an overhaul, and has done away with aberration tables. Now, if enabled, each image is examined for chromatic aberration and corrected automatically, with excellent results!]

Keith Nuttall, 2011

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