EXPLAINED — CHROMATIC ABERRATION
We break down the optical artefact that has photographers talking about it as though it’s a disease — why it occurs, and what it really means for your images
We break down the optical artefact that has photographers discussing it as though it’s a disease — why it occurs and what it really means for your images
While pixel-peepers who are keenly interested in the finer details of lens performance are often eager to delve deep into the physics of light that cause chromatic aberration, the term sees most shooters’ eyes glaze over.
So, hold the geekery, the jargon, and the heavy technical descriptions — here’s a simple and useful explanation.
Often referred to as ‘chromatic distortion’ or ‘spherochromatism’, chromatic aberration infiltrates otherwise picture-perfect images, making edges slightly softer than they should be or adding a telltale colour fringing around edges of subjects. These imperfections are optical artefacts formed by the failure of a lens to focus all colours to a single point.
At the risk of getting overly technical, here’s a quick rundown of how it works. A lens acts as a prism, bending light to a different angle, depending on its wavelength, each of which we see as a colour — just think of the image made famous by Pink Floyd’s iconic album art for Dark Side of the Moon, with colours passing through a lens emerging at different angles.
Light waves are comprised of three basic colours — red, blue, and green — and lenses have different refractive indices for each of these wavelengths. So, for your camera’s sensor to detect the combined colour of light, your lens needs to make all wavelengths of that particular ray hit the exact same point on your sensor.
This sounds simple enough, but, as various wavelengths strike a lens all at once, each of the rays will behave slightly differently depending on the glass that it’s passing through. So, the construction of your lens, your chosen focal length, and even the aperture that you’ve used may cause different wavelengths to arrive at points before or after where the focal plane sits, and, when this mismatch occurs, colours do not combine as they should.
To correctly align all these different light rays is a feat of engineering — and, in an
attempt to negate the effects of chromatic aberration, manufacturers often combine multiple low-dispersion (LD) glass elements in a lens array. If you were to pull most lenses apart, you shouldn’t be surprised to find upwards of 16 lens elements, each designed to correct different optical aberrations along the light’s short journey between the lens’ front element and the camera’s sensor. This LD glass minimizes the mis-registration of colours via an optical system. Telephotos have mostly benefited from LD glass, but, with their increased complexity, LD glass is widely used in standard and wide-angle lenses today.
Along with the rest of the Art series, the Sigma 24–35mm f/2 DG HSM Art’s complex lens array relies on a careful arrangement of LD glass. With one fluorite-like ‘F’ Low Dispersion (FLD) element, and seven Special Low Dispersion (SLD) glass elements — two of which are aspherical lenses — it boasts superlative image rendition, untarnished by residual chromatic aberration.
A Super Multi-Layer Coating is present to reduce flare and ghosting to ensure contrastrich and colour-neutral imagery — regardless of focus point or aperture setting, even in backlit conditions. In digital cameras, flare and ghosting may also be caused by reflections between the image sensor and lens surfaces. Here, too, Sigma’s Super Multi-Layer Coating is highly effective, assuring images of outstanding contrast — so it goes without saying that the image quality is very, very impressive.
Effectively covering three common lenses — the 24mm, 28mm, and 35mm — this lens isn’t your all-in-one zoom but accommodates the hugely popular wide focal lengths used for landscape and architectural photography, as well as environmental-type portraits. Plus, it’s an ideal solution for those looking to carry one or two fewer lenses in their kit.
For more information on Sigma’s range of lenses, visit crknz.co.nz.