Cam­era Col­lege: Aut­o­fo­cus sys­tems

How your cam­era fo­cuses, and how to get sharper re­sults

Digital Camera World - - CONTENTS -

To­day’s cam­eras put a com­pre­hen­sive set of aut­o­fo­cus con­trols at your fin­ger (and thumb) tips, but there are es­sen­tially two sim­ple steps you need to take re­gard­less of how so­phis­ti­cated or com­plex your cam­era is. First, set the fo­cus mode to match the sub­ject you’re shoot­ing, then de­cide where you want the cam­era to fo­cus.

There are typ­i­cally three aut­o­fo­cus or AF modes avail­able when you’re tak­ing pic­tures through the viewfinder of an SLR or CSC: a sin­gle-shot mode for sta­tion­ary sub­jects; a con­tin­u­ous ‘servo’ aut­o­fo­cus mode for mov­ing sub­jects; and an au­to­matic mode that switches be­tween the two as and when the cam­era de­tects some move­ment.

Sin­gle-shot aut­o­fo­cus is the mode to se­lect when you’re pho­tograph­ing sub­jects that aren’t mov­ing. It won’t let you take a pic­ture un­til the cam­era gets some­thing in fo­cus. Once it’s done this, the cam­era locks the fo­cus dis­tance in for as long as you keep a light pres­sure on the shut­ter re­lease but­ton, and the pic­ture will be sharply fo­cused un­less you or the sub­ject move. As the name sug­gests, con­tin­u­ous aut­o­fo­cus con­stantly ad­justs the fo­cus as long as you keep your fin­ger on the shut­ter re­lease. As a re­sult, it’s the best mode to use for pho­tograph­ing mov­ing sub­jects. The fo­cus po­si­tion is never locked, and the cam­era will let you fully press the shut­ter to take a pic­ture even if the sub­ject isn’t in fo­cus.

A cam­era’s aut­o­fo­cus or AF modes con­trol how the cam­era fo­cuses, but the AF points you see in the viewfinder de­ter­mine where the cam­era fo­cuses – and how pre­cise it is too. The greater the num­ber of AF

points there are and the wider they’re spread across the pic­ture, the eas­ier it is to fo­cus on sub­jects that aren’t in the cen­tre of the frame. A densely packed ar­ray of AF points can im­prove your chances of tak­ing sharp shots of sports and wildlife, as a mov­ing sub­ject is less likely to be in an area of the pic­ture that isn’t cov­ered by one of the points. It also means that you’re less likely to have to ad­just the cam­era po­si­tion to fo­cus be­fore re­com­pos­ing a shot.

You can choose to use just one of your cam­era’s aut­o­fo­cus points or all of them. With all of the AF points ac­tive, the cam­era will au­to­mat­i­cally choose the fo­cus point or points that cor­re­spond with the area it de­ter­mines should be in fo­cus. Typ­i­cally, this is the clos­est part of a scene or sub­ject, or the area of high­est con­trast . Au­to­matic fo­cus point se­lec­tion can be ef­fec­tive when you’re pho­tograph­ing a sub­ject against a clean back­ground, such as a bird fly­ing across a blue sky. How­ever, if that bird passes against a more de­tailed back­ground, there’s a chance that the fo­cus sys­tem will lock onto that in­stead.

Man­u­ally se­lect­ing an AF point gives you pre­cise con­trol over what’s in fo­cus, but it does mean that you have to keep the ac­tive AF point po­si­tioned over the sub­ject.

The num­ber of AF points and their pre­ci­sion is de­ter­mined by the lens at­tached to the cam­era – or rather, by its max­i­mum ef­fec­tive aper­ture.

A densely packed ar­ray of AF points can im­prove your chances of tak­ing sharp shots of sports and wildlife, as a mov­ing sub­ject is less likely to be in an area of the pic­ture that isn’t cov­ered by one of the points

In your cam­era man­ual, you might see an f/stop men­tioned in re­la­tion to the AF points, such ‘f/5.6 cross-type’ or ‘f/2.8 dual cross-type’. This is an in­di­ca­tion of the max­i­mum aper­ture re­quired to ac­ti­vate ad­di­tional lev­els of pre­ci­sion of a fo­cus­ing point. It’s the max­i­mum aper­ture of a lens that counts, not the aper­ture you set when you’re tak­ing a photo: an f/2.8 lens can en­able f/2.8-com­pat­i­ble sen­sors even if it’s used at f/11.

Some cam­eras have a cen­tre AF point that works with lenses that have a max­i­mum aper­ture as small as f/8. This comes in par­tic­u­larly handy when you’re us­ing a tele­photo lens with a tele­con­verter at­tached, as the ef­fec­tive max­i­mum aper­ture will be smaller than it is with the lens alone.

There are a dif­fer­ent set of aut­o­fo­cus op­tions avail­able when you’re us­ing an SLR’s Live View mode, or if you’re us­ing a mir­ror­less com­pact sys­tem cam­era (CSC) where Live View shoot­ing is the only op­tion. Here, the mir­ror as­sem­bly is locked out of the way – or in the case of a CSC, is non-ex­is­tent – so the cam­era’s imag­ing sen­sor is con­tin­u­ously ex­posed to the scene. This means that the cam­era has to use the im­age cap­tured by the sen­sor to drive the aut­o­fo­cus. This does mean that fo­cus­ing is po­ten­tially more ac­cu­rate, as what you see is what you get – with an SLR’s op­ti­cal viewfinder, the im­age you see is not the fi­nal im­age cap­tured by the cam­era’s imag­ing sen­sor.

Live View aut­o­fo­cus can in­clude such op­tions as face tracking, or blink or smile de­tec­tion. SLRs may also of­fer a ‘quick’ mode, which tem­po­rar­ily moves the mir­ror back into po­si­tion so that the cam­era’s AF mod­ule can be used to fo­cus, be­fore mov­ing the mir­ror out of the way again. It can be a bit of a clunky process, and not quite as quick as the name sug­gests.

It can be harder to keep the cam­era in po­si­tion – and the sub­ject in fo­cus – when us­ing the Live View dis­play, but a mir­ror­less cam­era equipped with an EVF en­ables you to sup­port the cam­era bet­ter, and im­proves your chances of tak­ing a sharp shot.


MISSED FO­CUS Use con­tin­u­ous aut­o­fo­cus when you pho­to­graph fast-mov­ing sub­jects, but be pre­pared to give your cam­era time to ‘warm up’ and track the ac­tion.

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