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

New Zealand D-Photo - - CONTENTS -

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.

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