All About lenses

A cam­era body is pretty well use­less with­out a lens, so how do these es­sen­tial com­po­nents of pho­tog­ra­phy ac­tu­ally work? In this first ar­ti­cle we cover all the key in­gre­di­ents of a pho­to­graphic lens’s op­ti­cal de­sign.

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The ear­li­est lenses were sim­ply a sin­gle el­e­ment of high-qual­ity glass ground and pol­ished into a spher­i­cal shape so the curved sur­faces bent the light rays which passed through them in or­der to fo­cus an im­age. More so­phis­ti­cated de­signs added more el­e­ments – both con­cave and con­vex in shape – to im­prove the op­ti­cal qual­ity across the im­age frame, and al­low for fo­cus­ing. Over time, op­ti­cal de­signs have be­come a lot more com­plex, in­volv­ing larger num­bers of el­e­ments ar­ranged in groups which can be moved in­de­pen­dently to fa­cil­i­tate fo­cus­ing and zoom­ing.

The key spec­i­fi­ca­tion of a cam­era lens is its fo­cal length which is ex­pressed in mil­lime­tres (although you’ll oc­ca­sion­ally see vin­tage lenses with the fo­cal length marked in cen­time­tres). The fo­cal length – or range, in the case of zooms – forms the lens’s model num­ber and is al­ways marked some­where on its ex­te­rior, ei­ther on the bar­rel or around the front, along with other key spec­i­fi­ca­tions such as the max­i­mum aper­ture.

The fo­cal length of a lens is ac­tu­ally the dis­tance from its op­ti­cal cen­tre – also called the rear nodal point – to the fo­cus­ing point in the cam­era which, ob­vi­ously, co­in­cides with the fo­cal plane in the cam­era (also called the plane of sharp fo­cus).

The fo­cal length of a lens con­trols the im­age size (or mag­ni­fi­ca­tion) and the an­gle-of-

view. Thus a short fo­cal length – which means the light rays have to be bent at more acute an­gles to fo­cus them at a point – has more wide-an­gle char­ac­ter­is­tics. A shorter fo­cal length lens there­fore pro­duces a closer or smaller im­age.

In a long fo­cal length lens, the light rays are bent at much shal­lower an­gles and so con­verge over a much greater dis­tance, giv­ing tele­photo char­ac­ter­is­tics. A longer fo­cal length lens there­fore pro­duces a larger (more mag­ni­fied) im­age. Tele­photo lenses are phys­i­cally longer than wide-an­gle types, and this is true of both zooms and primes. A prime lens has a fixed fo­cal length such as 24mm, 35mm, 50mm, 85mm, 200mm, etc. A zoom has a vari­able fo­cal length such as 24-70mm, 70200mm, 100-400mm, etc.

All these fo­cal lengths re­late to the 35mm for­mat, based on the frame’s di­ag­o­nal mea­sure­ment. If the sen­sor size or film for­mat is dif­fer­ent, a mag­ni­fi­ca­tion fac­tor comes into play and de­ter­mines an ef­fec­tive fo­cal length. This is par­tic­u­larly rel­e­vant to­day be­cause many dig­i­tal cam­eras have sen­sors which are smaller in size than a 35mm frame so there’s an in­crease to fo­cal length as it’s marked on the lens… as the im­age size will be big­ger on the smaller for­mat. The Mi­cro Four Thirds (M43) sen­sor for­mat has a 1.97x mag­ni­fi­ca­tion fac­tor com­pared to 35mm while the ‘APS-C’ size sen­sors have a 1.5x mag­ni­fi­ca­tion fac­tor (ex­cept for Canon cam­eras where it’s 1.6x). Thus an M43 lens with, say, a fo­cal range of 12-35mm is ef­fec­tively a 24-70mm (i.e. 12x2.0 and 35x2.0) and an APS-C lens with a fo­cal range of 16-60mm is ef­fec­tively a 24-90mm (i.e. 12x1.5 and 60x1.5). Like­wise, if you’re us­ing a 35mm for­mat lens on a cam­era with a smaller sen­sor (ei­ther di­rectly or via mount adap­tor), the ef­fec­tive fo­cal length will also in­crease. For ex­am­ple, a 24-70mm zoom on an M43 cam­era be­comes ef­fec­tively a 48-140mm and, on an ‘APS-C’ cam­era, be­comes a 36-105mm. This may all seem a bit con­fus­ing at first, but it’s re­lated to the size of the imag­ing area (specif­i­cally the di­ag­o­nal length), the stan­dard or nor­mal lens fo­cal length for this size, and its an­gle-of-view. There are var­i­ous math­e­mat­i­cal for­mu­lae in­volved which you don’t need to know. All you need to know is that lens fo­cal lengths in pho­tog­ra­phy are al­ways ex­pressed for the 35mm for­mat (film or dig­i­tal sen­sor) so if the imag­ing area is smaller, the ef­fec­tive fo­cal length will in­crease and, if its larger (as in dig­i­tal medium for­mat) it will de­crease.

An­other im­por­tant point to make here is that the fo­cal length of a lens doesn’t di­rectly con­trol per­spec­tive. The rea­son the two are so of­ten er­ro­neously linked is that per­spec­tive is re­lated to the dis­tance of the lens from the sub­ject and, in turn, this dis­tance can be a fac­tor of the fo­cal length (i.e. you may choose to move closer to a sub­ject when shoot­ing with a wider an­gle lens). How­ever, if a sub­ject was pho­tographed from the same dis­tance with both a tele­photo lens and a wide-an­gle lens, the per­spec­tive would be the same. True, the wide-an­gle lens will make the sub­ject seem fur­ther away, but if you en­larged the cen­tre of this im­age so it matched the tele­photo’s field-of-view, the two pic­tures would look ex­actly the same. Chang­ing the sub­ject dis­tance changes the per­spec­tive… i.e. the spa­tial re­la­tion­ship be­tween ob­jects in the fore­ground and back­ground of an im­age. Move in closer when us­ing a wide-an­gle lens, and the dis­tance be­tween the fore­ground and back­ground is ex­ag­ger­ated. Move fur­ther away from the sub­ject when shoot­ing with a tele­photo lens and the dis­tance be­tween the fore­ground and back­ground is com­pressed, an ef­fect known as fore­short­en­ing.

Zoom Lenses

The ear­li­est zoom – or vari­able fo­cal length – lens de­signs date back to the mid-1940s, but it wasn’t re­ally un­til the mid-1970s that mod­ern op­ti­cal de­sign and man­u­fac­tur­ing tech­niques al­lowed for more com­pact and af­ford­able zooms which de­liv­ered an ac­cept­able level of op­ti­cal per­for­mance. To­day, zoom lenses are more pop­u­lar than primes (lenses with a fixed fo­cal length) and range from ul­tra-wide-an­gle to ex­treme tele­photo.

As noted ear­lier, all lenses in­cor­po­rate a num­ber of el­e­ments – made from ei­ther op­ti­cal glass or op­ti­cal-grade resins – which bend (or re­fract) light to a point of fo­cus. Zoom lenses in­cor­po­rate an ex­tra set of el­e­ments to vary the fo­cal length, and this used to mean big and bulky de­signs with quite a num­ber of aber­ra­tions which de­graded the im­age qual­ity. Es­sen­tially, ev­ery ex­tra el­e­ment in a lens’s op­ti­cal con­struc­tion has the po­ten­tial to cre­ate ex­tra prob­lems which re­quire some form of cor­rec­tion. This used to be done by adding more el­e­ments, but ob­vi­ously this could also lead to more prob­lems, so it’s easy to see how zoom lens de­sign­ers faced real chal­lenges. A break­through came with the per­fect­ing of glass mould­ing tech­niques which has en­abled lens el­e­ments to be cre­ated, via com­puter-aided de­sign, to pro­vide ‘built-in’ cor­rec­tion. These are known as as­pher­i­cal el­e­ments which in­di­cates the sur­faces are shaped (i.e. not spher­i­cal) in or­der to cor­rect for var­i­ous aber­ra­tions. As­pher­i­cal el­e­ments are also of­ten made from op­ti­cal-qual­ity resins or, in some in­stances, use a spher­i­cal glass core over which is coated a resin to cre­ate the as­pher­i­cal sur­faces. These are known as hy­brid as­pher­i­cal el­e­ments.

As­pher­i­cal lens el­e­ments al­low com­plex zoom de­signs to now be achieved with a high de­gree of cor­rec­tion for var­i­ous aber­ra­tions, ex­cel­lent op­ti­cal per­for­mance, com­pact and light­weight con­struc­tions and com­par­a­tively low prices.

There are wide-an­gle zooms (17-35mm, for ex­am­ple), medium range zooms (28-105mm) and tele­photo zooms (100-300mm, 100-400mm, etc.). How­ever, there are also zooms with much longer fo­cal ranges such as 28300mm which spans wide-an­gle to tele­photo in one lens.

The down­side to zooms is that they are gen­er­ally slower than

a prime lens. Lens speed refers to the max­i­mum aper­ture or the max­i­mum amount of light the lens will let in to reach the imag­ing sen­sor. Be­cause of the ad­di­tional el­e­ments and the na­ture of a zoom’s op­ti­cal de­sign, more light is lost in­ter­nally and there are lim­its to the di­am­e­ter of the di­aphragm (which con­trols the aper­ture size). So, for ex­am­ple, a 24-70mm zoom may have a max­i­mum aper­ture of f2.8 at 24mm and f4.0 at 70mm whereas a 50mm prime lens may have a max­i­mum aper­ture of f1.4 or even f1.2 (re­mem­ber that the smaller f-num­bers in­di­cate a larger aper­ture open­ing).

As a gen­eral rule faster lenses are more ex­pen­sive be­cause of the mea­sures taken to in­crease their light trans­mit­ting ef­fi­ciency. A 24-70mm zoom with a con­stant max­i­mum aper­ture of f2.8 may well cost twice as much as one with the f2.8-4.0 max­i­mum aper­ture range just quoted.

Wide-An­gle Lenses

Tra­di­tion­ally, a wide-an­gle lens is one with a fo­cal length shorter than the 50mm so-called ‘stan­dard’ fo­cal length (all the fo­cal lengths quoted here re­late to the 35mm for­mat). How­ever, in re­al­ity, it’s any lens with a fo­cal length be­tween 20mm and 35mm.

As the fo­cal length de­creases, the an­gle-of-view in­creases and the sub­ject size be­comes smaller (rel­a­tive to the whole frame area be­ing recorded). Lenses with a fo­cal length shorter than 20mm – i.e. 14mm, 15mm or 19mm – are de­scribed as ‘ul­tra-wides’. Be­yond this, you move into the area of the fish-eye lens which has an an­gle- of-view of 180 de­grees and gives a dis­tinc­tive ‘bul­bous’ look. Ul­tra­w­ide an­gle lenses (and fish-eyes) need to be used with care as the de­gree of dis­tor­tion they pro­duce doesn’t work with all sub­jects.

The more ‘gen­eral pur­pose’ wide-an­gle fo­cal lengths are 21mm, 24mm or 28mm, with the first two pro­vid­ing a good bal­ance be­tween an ex­pan­sive an­gle-of-view (for land­scapes, street scenes, in­te­ri­ors, etc.) and prac­ti­cal­ity in pic­to­rial terms.

Wide-an­gle lenses have in­her­ently in­creased depth-of-field which means that when you fo­cus on a par­tic­u­lar point more of the scene in front of, and be­hind, this point will also be sharply ren­dered. Ul­tra-wide lenses ac­tu­ally have such long depth-of-field you don’t re­ally have to worry about fo­cus­ing at all. Depth-of-field is also var­ied ac­cord­ing to the se­lected aper­ture, with smaller aper­tures (i.e. f16 or f22) giv­ing much greater depth-offield than wider aper­tures (i.e. f2.8 or f2.0).

Tele­photo Lenses

Again, the def­i­ni­tion of a tele­photo lens is one with a fo­cal length greater (or longer) than 50mm, but the tele­photo range is gen­er­ally ac­cepted to be­gin at around 85mm which would be clas­si­fied as a ‘short tele­photo’ lens. Be­yond 300mm you move into the ‘su­pertele­photo’ range which in­cludes 400mm, 500mm and 600mm lenses. Any­thing longer than 600mm is gen­er­ally for spe­cialised ap­pli­ca­tions such as wildlife or some sports.

As a rule, im­age bright­ness de­creases as the fo­cal length in­creases so many tele­photo lenses or tele­zooms are de­scribed as be­ing ‘slow’ be­cause they may only have a max­i­mum aper­ture of f5.6 or smaller. Im­age bright­ness can be in­creased by in­creas­ing the di­am­e­ter of the aper­ture, but this re­sults in a bulkier and more ex­pen­sive lens. For ex­am­ple, a 300mm f2.8 lens may cost as much as $10,000, but a 300mm f5.6 lens could be as cheap as $500. Pro­fes­sional sports pho­tog­ra­phers need ‘fast’ tele­photo lenses, but as they’re earn­ing money from their pic­tures it’s eas­ier to jus­tify the in­vest­ment.

“As­pher­i­cal lens el­e­ments al­low com­plex zoom de­signs to now be achieved with a high de­gree of cor­rec­tion for var­i­ous aber­ra­tions.”

than 1/(fo­cal length) – so, for ex­am­ple, with a 300mm lens you shouldn’t se­lect a shut­ter speed slower than 1/300 se­cond or, with a 500mm lens, a speed slower than 1/500 se­cond. Of course, if you use a tri­pod, then you can se­lect slower shut­ter speeds, but al­ways make sure the tri­pod is se­curely set-up so there’s no pos­si­ble source of vi­bra­tion. A good idea is to use the self-timer to trig­ger the shut­ter (or a re­mote con­trol) so you can avoid ac­tu­ally press­ing the but­ton which could cause vi­bra­tions.

An in­creas­ing num­ber of tele­photo lenses in­cor­po­rate op­ti­cal im­age sta­biliser mech­a­nisms which are de­signed to cor­rect for a cer­tain amount of cam­era shake move­ment and al­low for hand-held shoot­ing at slower shut­ter speeds. Al­ter­na­tively, many mir­ror­less cam­era bod­ies have sen­sor-shift im­age sta­bil­i­sa­tion which also pro­vides cor­rec­tion for cam­era shake. Some sys­tems com­bine both in-cam­era and op­ti­cal im­age sta­bil­i­sa­tion to pro­vide en­hanced com­pen­sa­tion for cam­era shake.

Tele­photo lenses have an in­her­ently shal­low depth-of-field which means fo­cus­ing can be crit­i­cal if you need par­tic­u­lar parts of a scene to be re­pro­duced sharply.

Macro Lenses

The term ‘macro’ ap­pears quite fre­quently on mod­ern zoom lenses, but is ac­tu­ally of­ten used in­cor­rectly. The true def­i­ni­tion of a macro lens is one that fo­cuses suf­fi­ciently closely to the sub­ject to give a life­size re­pro­duc­tion (i.e. the ob­ject, or part of the ob­ject, is the same size in the im­age as it is in real life). Most so-called macro zooms are ac­tu­ally bet­ter de­scribed as ‘close fo­cus­ing’ and rarely give a re­pro­duc­tion ra­tio greater than 1:4 (i.e. quar­ter life­size).

In a true macro lens, the op­ti­cal de­sign is fully cor­rected for the aber­ra­tions which oc­cur when fo­cus­ing at such short dis­tances.

Macro lenses are gen­er­ally prime de­signs with fo­cal lengths of 50mm, 100mm or 200mm (in the 35mm for­mat). Closer fo­cus­ing ca­pa­bil­i­ties can also be achieved with ‘nor­mal’ lenses by us­ing ei­ther ex­ten­sion tubes which fit be­tween the cam­era body and the lens, or close-up ‘fil­ters’ which screw to the front of the lens.

Both are avail­able with var­i­ous mag­ni­fi­ca­tion pow­ers.

Per­spec­tive Con­trol (PC) Lenses

Per­spec­tive con­trol lenses – also called tilt-shift lenses be­cause of the way they work – are fairly spe­cialised tools, pri­mar­ily de­signed for ar­chi­tec­tural work, but with ap­pli­ca­tions else­where, in­clud­ing land­scape pho­tog­ra­phy.

The de­sign prin­ci­ple is the same as that of the tra­di­tional large for­mat film view cam­era which com­prised a lens stan­dard and a film stan­dard con­nected by a set of bel­lows. The flex­i­bil­ity of the bel­lows en­abled the lens and film to be tilted, swung or shifted in­de­pen­dently of each other, en­abling much greater con­trol over both per­spec­tive and sharp­ness than is pos­si­ble with a rigid-bod­ied cam­era.

How­ever, the same ef­fects can be achieved with a rigid-body cam­era fit­ted with a per­spec­tive con­trol (PC) or tilt-shift lens. These lenses in­cor­po­rate a mech­a­nism which al­lows the lens’s op­ti­cal axis – the im­age plane – to be moved in re­la­tion to the fo­cal plane (i.e. ei­ther tilted or shifted). A ver­ti­cal shift ad­just­ment al­lows for the cor­rec­tion of con­ver­gence which oth­er­wise makes tall build­ings ap­pear as if they’re top­pling over. Most PC lenses al­low for shifts to be made in ei­ther the ver­ti­cal or hor­i­zon­tal planes.

The tilt ad­just­ment al­lows the plane of sharp­ness to be ad­justed (lit­er­ally tilted from the nor­mal per­pen­dic­u­lar) which will give in­fi­nite depth-of-field with­out the need to se­lect smaller aper­tures. If you think of the plane of sharp­ness as a sheet of pa­per, held ver­ti­cally, then only the thick­ness of the pa­per rep­re­sents what would be sharply ren­dered in the im­age. If you then lay the sheet of pa­per down, which is the ef­fect of the tilt ad­just­ment, then ev­ery­thing cov­ered by it, from front to rear, would now be sharply ren­dered… which, in pho­to­graphic terms, rep­re­sents greater depth-of-field. Again, a tilt can be ap­plied in ei­ther the ver­ti­cal or hor­i­zon­tal plane. A tilt ad­just­ment that’s ap­plied in the hor­i­zon­tal pane is called a swing and en­ables sharp­ness con­trol in this plane.

In the next ar­ti­cle, we’ll look at some of the fea­tures of mod­ern lens de­sign in­clud­ing op­ti­cal im­age sta­bil­i­sa­tion, aut­o­fo­cus­ing drives, spe­cial el­e­ments and anti-re­flec­tion multi-coat­ings.

The op­ti­cal con­struc­tion of a pho­to­graphic lens com­prises el­e­ments set in groups. Shown here is the in­ner work­ings of Nikon’s AF-S 800mm f5.6E FL ED su­pertele­photo with the spe­cial el­e­ments shown in colour. All the el­e­ments work to­gether to fo­cus the light rays at the cam­era’s fo­cal plane while also cor­rect­ing for var­i­ous aber­ra­tions. Flourites ED Glass El­e­ments

Lens ‘model num­bers’ are made up of the fo­cal length (or fo­cal range for zooms) and max­i­mum aper­ture (or aper­ture range for zooms). In mod­ern lenses, the ex­tra sets of ini­tials re­fer to var­i­ous fea­tures such as op­ti­cal im­age sta­bi­liza­tion.

Here’s Nikon’s 800mm on the out­side. The fo­cal length of a lens is the dis­tance from its op­ti­cal cen­tre to the fo­cus­ing point in the cam­era. So long fo­cal length lenses – such as this su­pertele­photo – are go­ing to be phys­i­cally long too.

A mid-range zoom such as this 70200mm f2.8 is pop­u­lar be­cause it cov­ers a use­ful range of fo­cal lengths from short to mid tele­photo with the speed of a con­stant f2.8 max­i­mum aper­ture.

There’s even more size sav­ings to be had with Mi­cro Four Thirds tele­photo lenses such as the Le­ica DG Vario-El­mar 100-400mm 4.0-6.3. It’s the equiv­a­lent of a 100-800mm, but is com­pact enough to be used hand-held (as­sisted by op­ti­cal im­age sta­bil­i­sa­tion).

The ap­peal of mir­ror­less cam­eras with a smaller-than-35mm sen­sor is that it al­lows for more com­pact lenses. Olym­pus’s PRO se­ries 12100mm for its Mi­cro Four Thirds cam­era bod­ies is equiv­a­lent to a 24200mm, but in a lens that’s half the size of a full-35mm for­mat ver­sion.

Short fo­cal length lenses (which are also phys­i­cally much shorter) de­liver a wider an­gle-of-view which is why they’re known as wide-an­gle lenses. The ul­ti­mate wide-an­gle is the fish­eye which can de­liver an an­gle-ofview of up to 180 de­grees.

Long tele­photo lenses are in­her­ently big and heavy (due to the longer fo­cal length) which makes them more dif­fi­cult to han­dle. Ad­di­tion­ally, the higher mag­ni­fi­ca­tions in­crease the risk of cam­era shake which causes blur­ring of the im­age… as any tiny The Nikkor PC 19mm tilt-shift lens shown with a lateral shift ap­plied (right) to cor­rect for per­spec­tive in the hor­i­zon­tal plane; and a right swing (left) which will in­crease the zone of sharp­ness in the hor­i­zon­tal plane. Both these ad­just­ments can also be ap­plied in the ver­ti­cal plane.

move­ments dur­ing an ex­po­sure are also greatly mag­ni­fied. This is why you see sports pho­tog­ra­phers al­ways us­ing tripods or monopods to sup­port the cam­era and lens. If you want to hand-hold the lens, the gen­eral rule is that the shut­ter speed should not be slower

Per­spec­tive con­trol (PC) lenses al­low for the op­ti­cal axis to be shifted or tilted in re­la­tion to the im­age plane in or­der to give much greater con­trol over, re­spec­tively, per­spec­tive and sharp­ness.

Lens fo­cal lengths are ex­pressed in re­la­tion to the 35mm for­mat so there’s a mag­ni­fi­ca­tion fac­tor at play with lenses de­signed for smaller or larger size sen­sors. The Fu­ji­non GF 23mm f4.0 for Fu­ji­film’s GFX mir­ror­less dig­i­tal medium for­mat cam­era is de­signed for the 44x33 mm sen­sor so its ef­fec­tive fo­cal length is 18mm due to the rel­a­tive dif­fer­ence in the im­age sizes.

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