The curse of continuous auto-focus?

Published by Steve Litchfield at 7:16 UTC, February 6th 2012

Summary:

How's that for a controversial title? What I examine below is that there's more than one way to arrange focussing when shooting video on your smartphone - the rightly popular system of having continuous auto-focus does a good job a lot of the time but also manages to infuriate occasionally too. How bad is the problem, what are the alternatives and can I offer any tips for Symbian or Windows Phone users?

Having been something of a pioneer in phone-shot video over the years, it's fair to say that I lusted first after focus of any kind (the N93, shown above, was perhaps the first phone in the world to shoot usable video, back in 2006), then later after continuous auto-focus - after all, surely having the phone always working to keep the centre of frame in focus is a good thing, right? Well, not always...

Some of the various approaches to capturing video on phones over the years:

1. No focussing of any kind, lens is left focussed on the horizon. Examples include most phones and smartphones prior to about 2008.
     
2. No focussing, but the lens is left in a 'hyperfocal' position, i.e. ensuring maximum depth of field. Examples include the Nokia N86 and N8, with subjects from about 70cm to the middle distance being very crisp at all times.
     
3. Pre-focus, the user half presses the shutter button (or taps the screen, as appropriate) at the start of the clip and whatever's in the centre of the viewfinder becomes the focus subject. If you pan around and change the subject later in the clip, the original focus is retained - meaning that the new subject may be out of the depth of field for that lens position and so will appear blurry.
     
4. Continuous auto-focus, as it sounds, the software monitors the sharpness of whatever's in the viewfinder and if not happy with what it finds, steps in to 'experiment' with different focus positions until high enough sharpness is found again. This 'hunting' for focus whenever the subject is changed produces two effects: one, a slightly unsettling zoom in and out again (because the different focus positions all have slightly different fields of view); and two, an unsightly blurriness that clears up after a second or two once focus is restored.
     
5. Extended Depth of Field, also known as 'Full Focus', pioneered by Nokia in the phone world and using optical and electronic tricks to mean that everything from 40cm to infinity is crisp. This approach works surprisingly well, with the main disadvantage being that you can't film anything up really close (so no arty shots of small animals and flowers!). 

Although I've hinted above as to some of the pros and cons of each approach, options 2, 4 and 5 offer the best results for typical user-with-phone video capture. Typical subjects include:

What's interesting in the table above is that mode 4, continuous auto-focus, which you would think is going to win out overall by virtue of being more sophisticated, doesn't sweep the use-case board. Instead, modes 2 and 5 also do extremely well, neither of which involves any dynamic re-focussing of the camera phone optics. This turns out to be the achilles heel of mode 4 - the need to continuously monitor sharpness and adjust focal length to maximise it means that 'hunting' (as described above) is a real issue.

To illustrate 'hunting' and general continuous auto-focus behaviour, here's a test video, shot on the Nokia Lumia 800 with continuous auto-focus turned on. The bulk of the video includes side-by-side comparison with video capture of the same subjects with an EDoF-equipped smartphone, in this case the Nokia E6, typical of the breed. I'm not saying that the optics and sensor in the likes of the E6 are better than that in the Lumia 800 (or other a-f-enabled devices) - they're not, but the absence of 'hunting' does make for a less flawed experience for the video viewer:

 

 

Aside from me hopefully having just dispelled the myth that 'continuous auto-focus' is necessarily the best way to go (I'd argue that, clip for clip for casual users, EDoF video is best, as almost everything will always be in focus, etc.), it's worth pointing out that one way of having the best of both worlds would be to have the option of enabling or disabling continuous auto-focus, depending on subject and need.

This can, thankfully, be achieved on most smartphones. For example, on the Nokia N8, using the new Camera application, hyperfocal (mode 2, above) is the default, but you can also turn on 'Close-up' mode, in which continuous auto-focus is used to great effect. (In case you're wondering about the 'Close-up' name, it's because continuous auto-focus is only ever really needed for things closer than a few metres - the default hyperfocal system is fine for everything else.)

To take another example, behaviour is much the same on most Windows Phone devices. For example, on the HTC HD7 (and similar), continuous auto-focus can be simply turned off in settings, with 'off' reverting back to typical hyperfocal depth of field. Things are slightly more complicated on the Nokia Lumia 800, since, although auto-focus can be turned off manually, the lens is left in the last position used - so film something close-up with auto-focus on and then switch the feature off, and everything at normal distances will then be very blurry. (You thus have to make allowances if you want to switch focus modes, remembering what you last filmed and picking a subject at more typical distance in auto-focus mode, before then switching the feature off again.)

In summary, if your smartphone does have continuous auto-focus when capturing video, recognise that the first few seconds after switching subject will be subject to 'hunting'. So, for example, you're shooting some kids playing and then you swivel round to talk to mum or dad on-cam. Having turned, wait a couple of seconds before asking them whatever you were going to say. Recognise that these 'hunting' sections of video may have to be snipped out later in a video editor.

In practice, it's quite easy to simply allow an extra few seconds at the start of each planned clip 'in your head' - you probably won't be able to see the focus 'hunting' on your small smartphone screen, but the effect will be there and much more visible on your big desktop monitor or TV later on. So just allow for it and move on.

And consider turning continuous auto-focus off altogether - yes, it means a bit more fiddling around (especially if you have a Nokia Lumia device), but the results may be more consistent.

(And if you have a Symbian-powered EDoF-shooting phone then you can basically ignore everything I've just said, apart from not shooting subjects closer than about 40cm, since your video will always be auto-magically in focus anyway!)

Steve Litchfield, 6th Feb 2012, for All About Symbian and All About Windows Phone

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