Master your DSLR: Lens abberations

LEARN HOW LENS MANUFACTURERS ARE TACKLING OPTICAL PROBLEMS AND HOW YOU CAN GET SHARPER, BRIGHTER, CLEANER SHOTS USING LENS CORRECTION TOOLS.

[ TECHLIFE TEAM ]

THESE DAYS, CAMERA lenses are very well corrected for optical aberrations. The quality of corrective lens elements and multi-coatings mean that even zoom lenses with complex designs can produce results as crisp and colourful as older primes.

But that doesn’t mean they’re perfect. Chromatic aberration still continues to plague digital photos, for instance. This phenomenon is similar to the effect you see when light passes through a prism, with the different wavelengths of light becoming separated. In a digital image, this manifests as a coloured halo around high-contrast objects, and this can make images appear less sharp than they ought to be. More advanced lenses use lowdispersion elements that reduce the degree to which the different wavelengths are dispersed. Cheaper lenses don’t have these, and pictures taken with them will require more corrections when you process them.

Vignetting, where the corners of a picture are darker than the centre, is another common problem. It’s typically noticeable with fast wide-angle lenses, when they’re used at their maximum aperture. It’s caused by the aperture being so wide that it’s picking up the side of the

lens, too. Switching to a smaller aperture cleans this up nicely, but keeping some degree of corner shading is often a creative decision, particularly with portraits, as it keeps attention focused in the centre of the frame.

Vignetting can also be caused by stacking filters or an incorrect lens hood, something that’s easily remedied.

Lens hoods play an important role in reducing flare and ghosting. If you include a bright light source in the frame or allow stray light to glance across the front elements, you’ll end up with pictures that have a ghostly, contrast-reducing sheen across them, or polygonal hotspots. To get around this, fit a lens hood, use your hand to shield the lens and keep your lenses clean. If you have problems with chromatic aberration, vignetting and other problems, there’s a lot you can do to remedy these digitally. We’d recommend shooting raw, as you’ll be able to work with the full image data.