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Fol­low­ing our in­tro­duc­tory ar­ti­cle back in the Septem­ber/Oc­to­ber 2017 is­sue, we’re delv­ing a bit deeper into the de­sign of ac­ces­sory lenses to look at the lat­est tech­nolo­gies which are help­ing en­hance op­ti­cal per­for­mance and ex­tend ca­pa­bil­i­ties.

There has never been a more ex­cit­ing time in lens de­sign, with a greater choice of brands and models than ever be­fore. Much of this is be­ing driven by the mir­ror­less cam­era which, due to the shorter flange back dis­tance com­pared to an SLR, al­lows for greater flex­i­bil­ity in op­ti­cal de­signs. Mean­while the de­vel­op­ment of new tech­nolo­gies is en­abling spec­i­fi­ca­tions and ca­pa­bil­i­ties that could only be dreamt about a decade or so ago.

De­vel­op­ments in de­sign pro­cesses, more ad­vanced ma­te­ri­als (par­tic­u­larly en­gi­neer­ing plas­tics of var­i­ous types) and man­u­fac­tur­ing tech­niques have en­abled many of the tech­ni­cal chal­lenges which were re­stric­tive in the past to be over­come. These have al­lowed for more com­pact and light­weight lenses with­out com­pro­mis­ing per­for­mance, ei­ther op­ti­cally or me­chan­i­cally. Ad­di­tion­ally, more ‘ex­otic’ de­signs – in terms of the fo­cal length, zoom­ing range, lens speed or close-up ca­pa­bil­i­ties – can now be achieved… and, im­por­tantly, at af­ford­able prices.

So let’s ex­am­ine some key de­sign com­po­nents in a mod­ern cam­era lens which con­tribute to the ever im­prov­ing ca­pa­bil­i­ties, han­dling and per­for­mance. When light hits the sur­face of a lens el­e­ment some of it is trans­mit­ted and some of it is re­flected… or it would be if it wasn’t for spe­cial multi-lay­ered coat­ings. Not only would the re­flected light – be­tween four to ten per­cent at each lens sur­face – be lost to the ex­po­sure, it would also bounce around in­side the lens, cre­at­ing ghost­ing and flare which com­pro­mises both colour and con­trast. Multi-layer coat­ings – bet­ter known sim­ply as mul­ti­coat­ings – are de­signed to en­sure the widest pos­si­ble spec­trum of light wave­lengths are passed through the lens el­e­ments.

As the name sug­gests, these are mul­ti­ple coat­ings – ap­plied via a vac­uum de­po­si­tion process so they’re in­cred­i­bly thin – com­pris­ing

var­i­ous lay­ers each with a dif­fer­ent re­frac­tive in­dex, op­ti­mised to com­ple­ment the el­e­ment’s re­frac­tive in­dex. The re­frac­tive in­dex of an op­ti­cal ma­te­rial refers to its ef­fi­ciency at bend­ing – or re­fract­ing – light. The higher the re­frac­tive in­dex, the more ef­fi­cient it is at trans­mit­ting light.

In­ter­nal re­flec­tions within a lens be­came more of an is­sue with dig­i­tal cam­eras as sen­sor sur­faces are highly re­flec­tive and this has de­manded the de­vel­op­ment of more ef­fec­tive coat­ings. Like­wise mod­ern zooms which use a large num­ber of el­e­ments. A lot of work has gone into de­vis­ing ul­tra­thin coat­ings – at the nanome­ter level – which are also for­mu­lated to the de­sign of a par­tic­u­lar lens. The num­ber and type of coat­ings ap­plied to each el­e­ment’s sur­faces is in­di­vid­u­ally cal­cu­lated to match the lens type and the glass used.

Op­ti­cal Im­age Sta­bil­i­sa­tion

Blur caused by cam­era shake will ruin what might have oth­er­wise been a great pho­to­graph. Some­times it’s not fea­si­ble or de­sir­able to use a tri­pod to steady the cam­era, and this is where op­ti­cal im­age sta­bil­i­sa­tion in a lens proves its worth. The lens mak­ers all have dif­fer­ent names for their op­ti­cal im­age sta­bil­i­sa­tion sys­tems, but all es­sen­tially work along the same lines. (For the record, Canon in­tro­duced the first IS-equipped ac­ces­sory lens in a 75300mm tele­zoom, in 1995.)

Tiny gy­ros within the lens, known as an­gu­lar ve­loc­ity sen­sors, de­tect the small move­ments as­so­ci­ated with cam­era shake. This in­for­ma­tion is fed to a mi­cro­pro­ces­sor which trans­lates it into drive com­mands for the im­age sta­biliser it­self. This com­prises a small group of lens el­e­ments that can be moved in any di­rec­tion around the lens’s op­ti­cal axis to counter cam­era shake. It all hap­pens very rapidly and, with steadily more pow­er­ful mi­cro­pro­ces­sors and mi­cro­mo­tors, can now pro­vide up to four or five stops of cor­rec­tion for cam­era shake.

The rule-of-thumb for the ‘safe’ hand-hold­ing of a lens is that the slow­est shut­ter speed us­able equates to 1/fo­cal length… so, for ex­am­ple, this would be 1/200 sec­ond with a 200mm lens. Im­age sta­bil­i­sa­tion en­ables you to use slower speeds and, with four stops of cor­rec­tion, you could fea­si­bly go as slow as 1/15 sec­ond. In re­al­ity, it’s ad­vis­able to ex­per­i­ment as there are other fac­tors in­volved, such as the phys­i­cal chal­lenges of hold­ing a big­ger and heav­ier lens ab­so­lutely steady. But im­age sta­bil­i­sa­tion un­doubt­edly pro­vides ex­tra lee­way. It’s worth not­ing here that if you’re us­ing a cam­era with an ‘APS-C’ or Mi­cro Four Thirds size sen­sor, the 1/fo­cal length rule is still based on the ef­fec­tive (i.e. 35mm equiv­a­lent) fo­cal length.

A grow­ing num­ber of mir­ror­less cam­era sys­tems are com­bin­ing lens-based op­ti­cal sta­bil­i­sa­tion with cam­era-based sen­sor-shift sta­bil­i­sa­tion for even more ef­fec­tive cor­rec­tion of cam­era shake. The ba­sic prin­ci­ple of the lat­ter is the same, but in­stead the sen­sor is shifted around the cen­tre of the im­age to pro­vide the cor­rec­tion. Pana­sonic claims its lat­est ‘Dual I.S.’ com­bined sta­bil­i­sa­tion ex­tends the cor­rec­tion for cam­era shake up to 6.5 stops. It em­ploys a com­bi­na­tion of a gyro, ac­celerom­e­ter and data from the im­age sen­sor to more ac­cu­rately de­ter­mine the di­rec­tion of move­ment. The ex­tended cor­rec­tion range ef­fec­tively en­ables hand-held shoot­ing with shut­ter speeds as slow as one sec­ond, and ef­fec­tive fo­cal lengths up to 280mm.

Op­ti­cal im­age sta­bil­i­sa­tion first ap­peared in longer fo­cal length lenses, which made sense given their higher mag­ni­fi­ca­tion power also am­pli­fies any move­ments. It’s now be­com­ing in­creas­ingly com­mon in wider-an­gle lenses, ex­tend­ing the hand-held shoot­ing pos­si­bil­i­ties in low-light con­di­tions. The lat­est sta­bilis­ers can also au­to­mat­i­cally de­tect when the cam­era is mount­ing on a tri­pod, recog­nis­ing the ac­tion of panning and dis­abling the cor­rec­tion for move­ment in that di­rec­tion. This elim­i­nates the need to man­u­ally switch be­tween IS modes.

The ben­e­fits of im­age sta­bil­i­sa­tion aren’t lim­ited to en­abling the use of slower shut­ter speeds when shoot­ing hand-held. Al­ter­na­tively, it al­lows for the se­lec­tion of a smaller aper­ture – for ex­am­ple, f8.0 rather than f2.0 – which, in turn, pro­vides greater depth-of-field. There is also the pos­si­bil­ity of shoot­ing at lower ISO set­tings to op­ti­mise im­age qual­ity.

Ex­tra Low Dis­per­sion (ED) Op­ti­cal Glass

Vis­i­ble light is made up of the dif­fer­ent colours which all have their own wave­length within the vis­i­ble spec­trum. All these wave­lengths bend at slightly dif­fer­ent an­gles when they pass through a lens el­e­ment, which means they be­come dis­persed… in other words, they do not con­verge at the same point. You can see dis­per­sion at work when a beam of white light is passed through a prism and sub­se­quently splits into a rain­bow ef­fect.

In pho­tog­ra­phy dis­per­sion is a prob­lem as it cre­ates chro­matic aber­ra­tions which man­i­fest them­selves as colour fring­ing along high con­trast edges.

Chro­matic aber­ra­tions be­come harder to cor­rect as the lens fo­cal length in­creases.

His­tor­i­cally, Nikon was the first lens man­u­fac­turer to de­vise spe­cial for­mu­la­tions which cre­ated low-dis­per­sion char­ac­ter­is­tics for its op­ti­cal glass. Ox­ides of var­i­ous rare earth el­e­ments are used in the glass to min­imise the dif­fer­ences in the re­frac­tion of the colour wave­lengths. Sub­se­quent re­fine­ments have pro­duced ex­tralow dis­per­sion (ED) glass which is also called ul­tra-low dis­per­sion (UD) glass by some lens mak­ers (Canon, for ex­am­ple) and su­per­low dis­per­sion by oth­ers. These for­mu­la­tions have to be very pre­cisely cal­cu­lated so ED/UD glass is quite ex­pen­sive to make.

It is, how­ever, nowhere near as costly as cre­at­ing flu­o­rite el­e­ments. These have the low­est in­her­ent dis­per­sion char­ac­ter­is­tics of any op­ti­cal ma­te­rial, but they have to be painstak­ingly grown from syn­thetic flu­o­rite crys­tals. A num­ber of very high per­for­mance tele­photo lenses em­ploy flu­o­rite el­e­ments – they also have a very low re­frac­tive in­dex – but these are very ex­pen­sive in­deed. Many lens mak­ers have tried to come up with al­ter­na­tives, such as Sigma’s ‘F Low Dis­per­sion’ (FLD) glass – which was co-de­vel­oped with Hoya – claimed to have very sim­i­lar trans­mis­sion and dis­per­sion char­ac­ter­is­tics to flu­o­rite, but much cheaper to man­u­fac­ture.

It’s com­mon to see the term APO in a lens’s model num­ber and this is short for apoc­hro­matic (or apoc­hro­mat) which refers to any op­ti­cal de­sign in­cor­po­rat­ing spe­cial el­e­ments to cor­rect for chro­matic aber­ra­tions by fo­cus­ing the red, green and blue wave­lengths at the same point. APO lenses also cor­rect for spher­i­cal aber­ra­tions – where the light rays pass­ing through the edges of an el­e­ment con­verge at a dif­fer­ent fo­cal point to those pass­ing through the cen­tre, caus­ing a loss of sharp­ness par­tic­u­larly to­wards the edges of the frame – pri­mar­ily by em­ploy­ing as­pheric (or as­pher­i­cal) el­e­ments. As­pher­i­cal Lens El­e­ments As al­ready ex­plained, a lens el­e­ment works by re­fract­ing – or bend­ing – the light rays that pass through it, thanks to its spher­i­cal sur­faces. Put a num­ber of el­e­ments to­gether and they will bring these rays to­gether at a point… which fo­cuses the sub­ject. How­ever, the re­frac­tion process isn’t per­fect and var­i­ous lens aber­ra­tions – such as dis­tor­tion – are in­tro­duced along the way.

Orig­i­nally, this was cor­rected by adding more el­e­ments, but ob­vi­ously this made for big, bulky and ex­pen­sive lenses… and adding more el­e­ments cre­ated the po­ten­tial for more per­for­mance is­sues. The break­through came with non-spher­i­cal – or as­pher­i­cal – sur­faces, which es­sen­tially could be shaped to pro­vide ‘built-in’ cor­rec­tion for spher­i­cal aber­ra­tions in large-aper­ture lenses, and dis­tor­tion in wide-an­gle lenses.

An as­pher­i­cal lens el­e­ment cor­rects for dis­tor­tion and aber­ra­tions by con­tin­u­ally chang­ing the re­frac­tive in­dex from the cen­tre of the el­e­ment (i.e. its op­ti­cal axis) to the edges via non­spher­i­cal shap­ing of its sur­faces.

The first as­pher­i­cal el­e­ments were cre­ated us­ing the time­con­sum­ing process of se­lec­tive grind­ing and pol­ish­ing to cre­ate the more com­plex sur­face shapes. More re­cently, the re­fin­ing of glass mould­ing tech­niques (as well as com­puter-aided de­sign) has made it eas­ier – and much less ex­pen­sive – to pro­duce as­pher­i­cal el­e­ments. An­other pro­duc­tion tech­nique cre­ates what’s called a hy­brid as­pher­i­cal el­e­ment; this in­volves coat­ing a spher­i­cal core with an op­ti­cal resin to shape the sur­face. Lenses which in­cor­po­rate as­pher­i­cal el­e­ments some­times have the des­ig­na­tion ASPH in their model num­bers. Ul­tra­sonic Aut­o­fo­cus­ing Drives Aut­o­fo­cus­ing needs to be re­spon­sive and fast, which places con­sid­er­able de­mands on the me­chan­i­cal side of the process… i.e. ac­tu­ally mov­ing the group of el­e­ments that do the fo­cus­ing within the lens. Con­ven­tional elec­tric mi­cro­mo­tors sim­ply don’t get started quickly enough, so a com­monly-used al­ter­na­tive is a drive sys­tem which em­ploys ul­tra­sonic pulses.

This con­cept was pi­o­neered by Canon with its USM (Ul­tra Sonic Mo­tor) ring-type drive, but now ev­ery­body uses the same ar­range­ment, al­beit un­der a myr­iad of dif­fer­ent names – Silent Wave Mo­tor (SWM, Nikon), Su­per Sonic wave Mo­tor (SSM, Sony), Hy­per Sonic Mo­tor (HSM, Sigma), Su­per­sonic Wave Drive (SWD, Olym­pus), Ul­tra­sonic Silent Drive (USD, Tam­ron), and Su­per­sonic Drive Mo­tor (SDM, Pen­tax).

Ul­tra­sonic AF drives en­able a near-in­stant re­sponse fol­lowed by high-speed op­er­a­tion to en­hance ac­cu­racy. Ad­di­tion­ally, be­cause these sys­tems use ul­tra­sonic sound pulses that are be­yond our hear­ing range, aut­o­fo­cus­ing op­er­a­tion is also ex­tremely quiet.

Mod­ern lens de­signs em­ploy in­ter­nal fo­cus­ing (IF) which means the fo­cus­ing group (or groups) is lo­cated in the mid­dle of the lens in front of the di­aphragm and move in­de­pen­dently of all the other groups. The key ben­e­fits are that the lens’s length doesn’t change dur­ing fo­cus­ing, the front el­e­ment doesn’t ro­tate (im­por­tant when us­ing ori­en­ta­tion-sen­si­tive fil­ters such as po­laris­ers or grads), the fo­cus­ing group is lighter, thereby en­abling faster aut­o­fo­cus­ing and, in most cases, the whole op­ti­cal de­sign is more com­pact. Some lenses, par­tic­u­larly zooms, em­ploy a rear fo­cus­ing ar­range­ment where the fo­cus­ing group is lo­cated be­hind the di­aphragm, but the ben­e­fits are the same as for in­ner fo­cus­ing.

Tele­photo lenses of­ten have pro­vi­sions for lim­it­ing the fo­cus­ing range for faster AF op­er­a­tion when the sub­ject dis­tance doesn’t change sig­nif­i­cantly, and for mem­o­ris­ing a fo­cus­ing point which can be in­stantly re­set at the push of a but­ton. Weather Pro­tec­tion Cu­ri­ously, while weather-proofed cam­era bod­ies have been around for a while, sim­i­larly pro­tected lenses have only started ap­pear­ing com­par­a­tively re­cently. These de­signs em­ploy seals at the var­i­ous bar­rel tube junc­tions and a rub­ber gas­ket around the lens mount. In some cases, a spe­cial flu­o­rine coat­ing is used on the ex­posed sur­face of the front el­e­ment (and some­times the rear el­e­ment as well) to help re­pel mois­ture and also to al­low for eas­ier clean­ing.

The de­gree of weather seal­ing pro­vided can vary from brand to brand and across models, with the pre­cise de­gree of pro­tec­tion of­ten hard to pin down. It ranges from be­ing es­sen­tially only splash­proof through to the ca­pac­ity to with­stand longer-term ex­po­sure to con­stant rain or heavy sea spray. Ad­di­tion­ally, not all so-called

“the de­gree of weather seal­ing pro­vided can vary from brand to brand and across models with the pre­cise de­gree of pro­tec­tion of­ten hard to pin down.”

weather-pro­tected lenses al­low for op­er­a­tion in sub-zero tem­per­a­tures (which, amongst other things, af­fects the lu­bri­cants) so there could be is­sues when shoot­ing, par­tic­u­larly for pro­longed pe­ri­ods, in the snow or in icy con­di­tions. Mi­cro­pro­ces­sors To­day’s lenses are as much elec­tronic de­vices as they are op­ti­cal ones, given so much of what they do is han­dled by one or more high-speed mi­cro­pro­ces­sors.

As a ba­sic level, the mi­cro­pro­ces­sor de­liv­ers in­for­ma­tion about the lens (fo­cal length/range, aper­ture range and the min­i­mum fo­cus­ing dis­tance) back to the cam­era body. This de­ter­mines AF op­er­a­tion, ex­po­sure con­trol and, in­creas­ingly, in-cam­era cor­rec­tions for lens aber­ra­tions, in­clud­ing dis­tor­tion and vi­gnetting. Most dig­i­tal cam­eras, mir­ror­less or D-SLR, are per­form­ing some level of lens cor­rec­tion on-the-fly at the point of cap­ture, in ad­di­tion to al­low­ing for man­ual se­lec­tion of cer­tain func­tions.

In-lens pro­ces­sors also con­trol the AF drive and han­dle an im­age sta­biliser’s op­er­a­tion.

More re­cently, some lens mak­ers – most no­tably Sigma and Tam­ron – are of­fer­ing the fa­cil­ity to cus­tomise these op­er­a­tions via a USB in­ter­face or dock and ded­i­cated soft­ware. Sigma’s USB Dock is es­sen­tially a lens mount adapter which en­ables con­nec­tion to a com­puter via a USB ca­ble. Once this is done, the Sigma Op­ti­mi­sa­tion Pro soft­ware al­lows for firmware up­grades as well as ad­just­ments to the aut­o­fo­cus­ing speed and dis­tance lim­iter range, cor­rec­tions for front/back fo­cus­ing, changes to the im­age sta­bil­i­sa­tion as mon­i­tored in the viewfinder, and the abil­ity to set the cus­tom oper­at­ing modes for the lenses which have this ca­pa­bil­ity (and which can be tai­lored to spe­cific sub­jects or sit­u­a­tions).

As al­ter­na­tives to the tra­di­tional cut­ting and pol­ish­ing (time­con­sum­ing and ex­pen­sive pro­cesses), the as­pher­i­cal el­e­ments in mod­ern lenses are made in a num­ber of ways, in­clud­ing us­ing glass mould­ing tech­niques or by ap­ply­ing an op­ti­cal resin over a spher­i­cal glass core to cre­ate what’s called a hy­brid as­pher­i­cal el­e­ment.

The dif­fer­ent coloured re­flec­tions ev­i­dent on the front el­e­ment of Le­ica’s new APO Sum­mi­cron-SL 75mm f2.0 ASPH prime lens in­di­cate the dif­fer­ent re­frac­tive in­dices of the var­i­ous multi-coat­ing lay­ers. Multi-coat­ings in­crease the trans­mis­sion ef­fi­ciency by min­imis­ing re­flec­tions. Note that Le­ica uses the ‘APO’ des­ig­na­tion in this lens’s model num­ber which is short for apoc­hro­matic, and means the op­ti­cal de­sign in­cor­po­rates cor­rec­tions for both chro­matic and spher­i­cal aber­ra­tions.

Now that lenses con­tain one or more mi­cro­pro­ces­sors, there’s the op­tion of tweak­ing spec­i­fi­ca­tions or mak­ing firmware down­loads. Sigma’s USB Dock fits to the lens mount and is then plugged into a com­puter via a USB ca­ble. Many ad­just­ments can then be made via pro­pri­etary soft­ware.

Ul­tra­son­i­cally-pulsed drives are now widely used for aut­o­fo­cus­ing, as they are more re­spon­sive (hav­ing less in­er­tia than a con­ven­tional elec­tric mo­tor) and also nearly to­tally silent. Each lens maker has their own ti­tle for what is es­sen­tially the same sys­tem… Tam­ron’s is called the ‘Ul­tra­sonic Silent Drive’ (USD for short).

A lot of in­for­ma­tion now passes be­tween lens and cam­era body via var­i­ous con­fig­u­ra­tions of elec­tri­cal con­tacts, as seen here on Zeiss’s ZF mount on the Otus 28mm f1.4 lens.

Weather pro­tec­tion mea­sures also in­clude a gas­ket around the lens mount, as shown here on one of Fu­ji­film’s GF mount lenses for the dig­i­tal medium for­mat GFX 50S.

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