Im­age sta­bil­i­sa­tion: elim­i­nat­ing cam­era shake

iPhone cam­eras are sur­pris­ingly good at cut­ting out wob­bles – but how?

Mac Format - - CONTENTS - Adam Banks

The Steadicam is a Hol­ly­wood sta­ple, but not much good for home movies when cine cam­eras be­came small enough to hold in one hand, they be­gan to change the way we recorded fam­ily mem­o­ries, as well as free­ing movie di­rec­tors from the pedestals on which bulky film­ing rigs were wheeled around.

But there was a vis­i­ble prob­lem: while most peo­ple could hold a stills cam­era – well, still for long enough to take a photo – shoot­ing mov­ing pic­tures re­sulted in what be­came known as ‘shaky­cam’.

While the pro­duc­ers of gritty dra­mas shrugged and called it a style, au­di­ences – in­clud­ing your in-laws – were in­clined to grum­ble that they felt sea­sick and couldn’t see what was go­ing on. In the mid-1970s, cam­era­man Gar­rett Brown in­vented a body­mounted, coun­ter­weighted gim­bal sys­tem that used ten­sioned springs to trans­fer the op­er­a­tor’s move­ments to the cam­era with­out the jit­ter. Mar­keted as the Steadicam, it be­came a Hol­ly­wood sta­ple, but a rig cost­ing tens of thou­sands of dol­lars and re­quir­ing weeks of train­ing didn’t of­fer much hope for the qual­ity of home movies.

Gy­ro­scopic sen­sors

In the 1990s, SLR cam­era mak­ers in­tro­duced sta­bilised lenses that used gy­ro­scopic sen­sors to de­tect small move­ments and elec­tro­mag­nets to move a lens el­e­ment to com­pen­sate. Orig­i­nally de­signed to pre­vent blur due to cam­era shake in still pho­tos, the same tech­nol­ogy – gen­er­ally re­ferred to as op­ti­cal im­age sta­bil­i­sa­tion (OIS) – can be used for video, although the re­sults are less pre­dictable. More re­cently, mir­ror­less cam­era mak­ers have be­gun ap­ply­ing sta­bil­i­sa­tion to the im­age sen­sor. This can work with un­sta­bilised lenses and po­ten­tially al­lows larger move­ments to be elim­i­nated.

Sta­bil­i­sa­tion is de­scribed as op­er­at­ing in two to six axes of move­ment; be­yond the three di­men­sions (or X, Y and Z axes), this refers to han­dling trans­la­tional mo­tion – where the cam­era moves up, down, left, right, back­wards or for­wards – as well as an­gu­lar mo­tion, where it tilts, yaws or rolls. Minia­turised sys­tems like those in iPhones and small drones use servo mo­tors, while bulkier gim­bals use smoother brush­less mo­tors.

Elec­tronic sta­bil­i­sa­tion

As dig­i­tal sen­sors took over from film, elec­tronic sta­bil­i­sa­tion also be­came fea­si­ble. The cam­era’s soft­ware com­pares each frame to the last to find shifts rep­re­sent­ing small un­in­ten­tional move­ment or vi­bra­tion, then

shifts the pix­els back. The same tech­nique can be used in post-pro­duc­tion us­ing fea­tures like Fi­nal Cut Pro X’s Sta­bi­liza­tion and Adobe Pre­miere Pro’s Warp Sta­bi­lizer.

Dig­i­tal sta­bil­i­sa­tion can’t quite match the re­sults of its op­ti­cal coun­ter­part: it doesn’t pre­vent mo­tion blur af­fect­ing each frame, and larger amounts of move­ment still pro­duce a rolling shut­ter ef­fect: the sen­sor records each pixel in turn, and the time dif­fer­ence between the top left and bot­tom right of the frame dis­torts mov­ing ob­jects, cre­at­ing a wob­bling ‘jelly’ ef­fect when the cam­era also moves.

Even so, be­cause it doesn’t re­quire any spe­cial cam­era hard­ware, only pro­cess­ing power, dig­i­tal sta­bil­i­sa­tion has be­come com­mon­place in phones, helped along by dig­i­tal sig­nal pro­ces­sor (DSP) chips. It’s the rea­son why clips from your iPhone look a lot stead­ier than those from old film reels or video­tapes. But op­ti­cal sta­bil­i­sa­tion can do even bet­ter – if it can be squeezed in.

First for phone cam­eras

Nokia’s Lu­mia 920, re­leased in 2012, was the first phone cam­era with op­ti­cal im­age sta­bil­i­sa­tion, which helped both stills and video. But the 920 was crit­i­cised for be­ing big and heavy, and as phones got slim­mer and lighter, find­ing room for phys­i­cal sta­bilis­ers got harder. It was with the launch of 2014’s over­sized 6 Plus that the iPhone first of­fered OIS. An Ap­ple patent ap­pli­ca­tion filed that year (granted in 2017 as US9591221B2) de­scribes a coil mount­ing sys­tem that uses tiny elec­tro­mag­nets to shift the lens. Ini­tially this was only op­ti­mised for still im­ages, but with the next year’s 6s Plus it was ap­plied to video as well.

Sub­se­quent Plus mod­els and the iPhone X use sim­i­lar sys­tems, with the iPhone X be­ing the first to have dual OIS, whereas only the wide-an­gle lens in the 7 Plus and 8 Plus has op­ti­cal sta­bil­i­sa­tion. One catch is that the OIS sys­tem can’t be turned off when it isn’t re­quired. Users have found that when used with ex­ter­nal mounts that elim­i­nate shake, such as a drone or hand­held gim­bal, Ap­ple’s OIS makes the cam­era more sus­cep­ti­ble to small vi­bra­tions rather than less. Some have re­sorted to lock­ing the iPhone’s lens in place with a mag­net. This could af­fect the hard­ware over time, although iPhone ac­ces­sories com­monly in­cor­po­rate mag­nets, so the risk is likely small.

In 2016, Chi­nese man­u­fac­turer Oppo an­nounced SmartSen­sor, a sys­tem for sen­sor sta­bil­i­sa­tion in phones, claim­ing it’s more pre­cise and en­ergy-ef­fi­cient than lens-based OIS. We haven’t seen it catch on so far, though.

Op­ti­cal im­age sta­bil­i­sa­tion in iPhones at­tempts to do the same job as this Steadicam rig, with tiny mo­tors.

Ap­ple’s 2014 patent ap­pli­ca­tion il­lus­trates how mag­nets and springs are fit­ted around the lens to shift it in re­sponse to move­ment de­tected by gy­ro­scopic sen­sors.

Both of the iPhone X’s rear cam­eras have OIS. A longer fo­cal length mag­ni­fies shake, so the tele­photo has seven elec­tro­mag­nets in­stead of four.

In this photo of the iPhone 8 Plus’ cam­eras, you can see the ex­tra mount on the wide-an­gle cam­era (right). Sam Lion­heart/iFixit CC BY-NC-SA

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