What makes a good lens?

Canon L lens tech­nol­ogy, ex­plained sim­ply.

HWM (Singapore) - - Learn - By Alvin Soh

The Canon L se­ries of lenses is marked by a bright red line on the lens bar­rel, and it’s the lens of choice for pro­fes­sion­als us­ing Canon cam­eras. A lot goes into the mak­ing of an L lens, so that the nal im­age is sharp, clear, and free from op­ti­cal dis­tor­tions. Here are key L tech­nolo­gies, sim­ply ex­plained.

1 AS­PHER­I­CAL LENSES MIN­I­MIZE ABERRATIONS

Spher­i­cal lenses are cheaper to pro­duce, but be­cause light pass­ing through a spher­i­cal lens con­verges at dif­fer­ent fo­cal points, it re­sults in a softer, low con­trast im­age, like pho­tograph­ing through a sheer veil. This is an op­ti­cal phe­nom­ena known as spher­i­cal aber­ra­tion.

An as­pher­i­cal lens, which has a non-spher­i­cal sur­face, is de­signed to con­verge light rays onto a sin­gle fo­cal point, al­low­ing the cam­era to cap­ture high con­trast im­ages with uni­form sharp­ness. As­pher­i­cal lenses can also re­duce other op­ti­cal aberrations like astig­ma­tism and dis­tor­tion.

As­pher­i­cal lenses, how­ever, are difcult to make. Some of Canon’s as­pher­i­cal lens de­signs re­quire pre­ci­sion tol­er­ances that are in the nanome­ter range, or 1/1,000,000 of a mil­lime­ter. De­vi­a­tions of more than that are re­jected.

2 GLASS ISN’T AL­WAYS BEST

Chro­matic aber­ra­tion, also known as color fring­ing or pur­ple fring­ing, is when im­ages look blurry, or when col­ors ap­pear around the edges of ob­jects, es­pe­cially in high­con­trast and back­lit pho­to­graphs.

Chro­matic aber­ra­tion oc­curs be­cause the wave­lengths of light bend at dif­fer­ent points within a lens, and the

lens is un­able to fo­cus all wave­lengths onto the same fo­cal plane.

To counter chro­matic aber­ra­tion, Canon de­vel­oped lenses made from syn­thetic crys­tals, com­posed mainly of cal­cium uoride. Flu­o­rite lenses have bet­ter light dis­per­sion prop­er­ties than reg­u­lar glass, which helps to re­duce chro­matic aber­ra­tion, and also pro­duces clearer im­age de­lin­eation than con­ven­tional glass. 3 UL­TRA­SONIC MO­TORS FOR UL­TRA­SONIC AF Al­most all L lenses use Canon’s Ul­tra­sonic Mo­tor (USM), the com­pany’s unique drive sys­tem. The USM con­verts ul­tra­sonic vi­bra­tion to ro­ta­tional en­ergy with one-thou­sandth of a mil­lime­ter pre­ci­sion, and draws min­i­mal power from the cam­era while re­main­ing quiet.

The USM has high torque, with vir­tu­ally in­stan­ta­neous starts and stops. All of its qual­i­ties make it ideal as a high­speed, high-pre­ci­sion mo­tor for the AF sys­tem. 4 OIS HOLDS THE SHOT STEADY Op­ti­cal im­age sta­bi­liza­tion (OIS) is crit­i­cal for get­ting sharp and steady im­ages, es­pe­cially at long fo­cal lengths where small nudges trans­late to big move­ments.

Canon’s OIS tech­nol­ogy de­tects cam­era shake with a vi­bra­tion gyro sen­sor and shifts parts of the lens to coun­ter­act the im­age blur.

While Canon’s IS tech­nol­ogy is lens-based, there are also cam­eras that come with in-cam­era body OIS. There are pros and cons to ei­ther ap­proach, but gen­er­ally speak­ing, lens-based OIS is more ne-tuned, as it’s tai­lored specically for the lens it’s housed in. Lens-based OIS can also be more ef­fec­tive in com­pen­sat­ing for the more vi­o­lent shakes from long tele­photo lenses. 5 COAT­INGS RE­DUCE GHOST­ING AND FLARE Flare or ghost­ing op­ti­cal aberrations can oc­cur when light reects off the lens or sen­sor. Canon’s L lenses con­tain a com­bi­na­tion of unique coat­ings to com­bat these aberrations.

This in­cludes the newly de­vel­oped Sub­wave­length Struc­ture Coat­ing (SWC), which min­i­mizes are and ghost­ing by us­ing wedge­shaped nano struc­tures.

Air Sphere Coat­ing (ASC) ap­plies a lm con­tain­ing mi­cro­spheres of air over the va­porde­po­si­tion lens coat­ing, re­sult­ing in ex­cep­tional anti-reec­tiv­ity. Su­per Spec­tra Coat­ing, which al­lows up to 99.9% of light to emerge from each lens el­e­ment, de­liv­ers op­ti­mum color bal­ance, so that col­ors are re­pro­duced ac­cu­rately.

Pho­tographed with EF24-105mm

Pho­tographed with EF16-35mm f2.8L III USM

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