WHY CANON’S LAT­EST CAM­ERAS FO­CUS SO QUICKLY

Dual Pixel CMOS AF ex­plained.

HWM (Malaysia) - - LEARN - by Mar­cus Wong

Ask any pho­tog­ra­pher what their top con­sid­er­a­tions are when se­lect­ing a cam­era, and chances are you’ll find aut­o­fo­cus (AF) speed and ac­cu­racy right up there. Af­ter all, all the res­o­lu­tion in the world won’t help if the im­age is out of fo­cus.

This is only more ev­i­dent now when In­ter­change­able Lens Cam­eras are in­creas­ingly be­ing used to take videos as well as stills, and no­body wants footage that is out of fo­cus. And that’s where Canon’s Dual Pixel CMOS AF stands out, as it prom­ises fast AF even dur­ing Live View and Movie Record­ing mode.

Aut­o­fo­cus ba­sics

First, an AF primer. AF meth­ods for to­day’s cam­eras gen­er­ally fall into two meth­ods – Con­trast De­tec­tion AF, and Phase De­tec­tion AF.

Con­trast De­tec­tion AF works by search­ing for the point of max­i­mum con­trast, as that’s when an im­age is per­fectly in fo­cus. So, the cam­era es­sen­tially moves the lens back­wards and for­wards, mea­sur­ing the con­trast at each point. When the cam­era has de­tected that it has just passed the point of high­est con­trast, it tracks back a step and takes the pic­ture. That’s why you see the cam­era hunt­ing for fo­cus when you’re try­ing to fo­cus in low light or on sub­jects that have low con­trast – like sheep in dense fog, for ex­am­ple. Ob­vi­ously, that’s not ideal for video work when all that shift­ing back and forth gets recorded live.

With Phase De­tec­tion AF though, ev­ery AF point will have a cor­re­spond­ing pair of sen­sors lo­cated on a Phase De­tec­tion

AF mod­ule that’s typ­i­cally lo­cated be­low the re­flex mir­ror. When you trig­ger AF; light from the lens bounces off the re­flex mir­ror down to the sen­sors be­low. One sen­sor will read what’s cap­tured from one side of the lens, while the other will read what’s cap­tured from the other side, and the im­ages on both sides will be com­pared.

If the im­ages are in phase, then the ob­ject is in fo­cus. Con­versely, if the im­age is not in fo­cus, the dif­fer­ence in phase tells the cam­era just how much and in which di­rec­tion it needs to ad­just for per­fect fo­cus. That’s why Phase De­tec­tion is faster, be­cause the cam­era just needs to get one set of read­ings in­stead re­ly­ing on a con­tin­u­ous process of trial and er­ror.

The prob­lem

Typ­i­cal DSLR (Dig­i­tal Single-Lens Re­flex) cam­era sys­tems use the re­flex mir­ror to re­di­rect light both to the viewfinder and to a sep­a­rate Phase De­tec­tion AF mod­ule. This works fine with stills, but when the re­flex mir­ror is raised for Live View mode or Movie record­ing mode, that same Phase De­tec­tion AF mod­ule is taken out of the equa­tion be­cause the re­flex mir­ror is raised out of the way.

This means the only op­tions left is to ei­ther use the imag­ing sen­sor it­self to per­form Con­trast De­tec­tion AF or to add sep­a­rate Phase De­tec­tion AF sen­sors on the imag­ing sen­sor – what’s com­monly known as Hy­brid AF.

How­ever, adding sep­a­rate Phase De­tec­tion AF sen­sors eats into the amount of space the imag­ing sen­sor has to gather light, so the amount of cov­er­age pos­si­ble on Hy­brid AF sys­tems has gen­er­ally been small, lim­it­ing their ef­fec­tive­ness.

Canon’s so­lu­tion

What Canon has done with Dual Pixel CMOS AF is to take an area on the sen­sor that’s about 80 per­cent of the width and 80 per­cent of the height of the en­tire im­age plane, and cover it with Phase De­tec­tion AF pix­els, each of which com­prises two in­de­pen­dent pho­to­di­odes.

When the cam­era is fo­cus­ing, each pho­to­di­ode cap­tures light sep­a­rately and hence the pair acts as a Phase De­tec­tion AF sen­sor, get­ting the nec­es­sary in­for­ma­tion to let the cam­era know how much and in which di­rec­tion to ad­just fo­cus. When you press the shut­ter to take the shot, this light in­for­ma­tion is then com­bined again and the Phase De­tec­tion AF pixel acts like any reg­u­lar pixel, so no light is lost for imag­ing.

This means no more footage of your cam­era hunt­ing back and forth to get fo­cus as you won’t have to rely on Con­trast De­tec­tion AF, and also that fo­cus shift can be done more seam­lessly. With al­most ev­ery part of the im­age hav­ing a cor­re­spond­ing AF point, switching fo­cus is sim­ply a mat­ter of choos­ing which AF point to use - the cor­re­spond­ing Phase De­tec­tion AF pix­els will do the rest.

In essence, Dual Pixel AF gives the best of both worlds when us­ing the cam­era to cap­ture stills and video with the mir­ror up.

Hy­brid aut­o­fo­cus is so termed be­cause it com­bines con­trast and phase-de­tec­tion AF for dif­fer­ent tasks. How­ever, tra­di­tion­ally, phase-de­tec­tion and imag­ing pix­els were in­de­pen­dent of each other on a sen­sor.

In Canon’s Dual Pixel CMOS sen­sor, each pixel is split into two in­de­pen­dent light col­lec­tion diodes, which func­tions for both phase-de­tec­tion AF and imag­ing.

Dur­ing fo­cus­ing, light col­lected in each diode is com­pared for phase-de­tec­tion AF cal­cu­la­tions. When ex­po­sure is taken, the light on both sides are com­bined and treated as one im­age pixel.

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