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Daily FTU hints and tips: Live view autofocus permutations

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No.4: What you need to know about live view autofocus and which bodies work best with which lenses.

Live view means being able to see what the lens on the camera sees, viewed either via an external screen, or an eye-level electronic viewfinder. Live view is not, as far as I know, officially part of the Four Thirds standard. Panasonic and Olympus have developed their own solutions, but thankfully they do embrace the issue of compatibility - up to a point.

Olympus got the first crack of the whip with Four Thirds system live view via the E-330, introduced in early 2006. The E-330 was the first 'conventional' DSLR with full colour live view. Technically, older Olympus designs like the E-10 and E-20 were reflex cameras with live view, but these had non-interchangeable lenses. The E-330's half sibling, the Panasonic Lumix DMC-L1, was launched a little later, and it shared much of the E-330's internals, but had a completely different style body.

Even at this early stage there were detail differences in live view implementation between Olympus and Panasonic. The Olympus E-330 featured a novel secondary sensor to enable the use of conventional DSLR fast phase detect AF sensors during live view. The problem with this system was that it was expensive and required a complicated viewfinder system.

Panasonic decided against the use of the E-330's secondary sensor system, relying instead on dropping the reflex mirror down momentarily, interrupting live view, when AF was in action. Neither employed contrast detect AF, they system almost all compact cameras use, which relies on the camera's main sensor to determine focus.

What is the difference between phase detect and contrast detect AF?

Phase detect: Put simply, conventional DSLRs (and autofocus film SLRs) use a rangefinder system fed by light diverted off the camera's semi-slivered reflex mirror. The sensors can work out which way the focus needs to be changed and even estimate how far the focus needs to be adjusted. This system is fast and responsive, but the camera's reflex mirror needs to be deployed, which is fine for viewfinder use, but not for live view. During live view the mirror needs to be pulled up to enable the lens to illuminate the camera's main sensor.

Contrast detect: When an image is out of focus, contrast is low. Contrast will be at its highest at the point of best focus. So contrast detect AF analyses contrast and knows that it's moving focus in the right direction if the contrast increases with focus change. However, the system will only know that AF has been reached once contrast starts to drop as the point of best focus has been over-shot. The system then back tracks to the focus position that returned the highest contrast, and so the best focus. It's very accurate - more accurate than phase detect AF -but slower. It's ideal for live view as it works while live view is active and does not interrupt live view.

Panasonic and Olympus have worked together to develop full-time live view AF. It required a new generation of lenses with AF motors optimised for full-time live view. The gearing of older lenses was optimised for fast changes in focus from one end of the scale to the next. Live view AF requires precise, slower, focus changes.

Panasonic's live view system has been dedicated entirely to full time live view optimised AF lenses and works with both Olympus and Panasonic lenses designed with this optimisation.

Here are the lenses, at the time of writing, that are optimised for live view:

Panasonic Leica:
14-50mm f/3.5-5.6
14-150mm f/3/5-5.6
25mm f/1.4

Olympus Digital Zuiko:
9-18mm f/4-5.6
14-42mm f/3/5-5.6
14-54mm f/2.8-3.5 II
25mm f/2.8 pancake
40-150mm f/4-5.6
70-300 f/4-5.6

Only using Olympus bodies starting with the E-420 and E-520 can you autofocus Four Thirds lenses not optimised for full time live view, and that includes Olympus Digital Zuiko SWD lenses. This is because Olympus developed a so-called 'hybrid' live view AF system. When fitted with lenses not optimised for live view AF, Olympus camera models equipped with hybrid live view AF use contrast detect AF in continuous live view mode, but when you press the shutter release home a final phase detect focus check is made before the picture is taken.

This involves dropping the mirror, adjusting focus as necessary, then lifting the mirror to take the picture. It adds significantly to the shutter response lag.

Why does hybrid AF mode need to do a phase detect AF action at all? Because contrast detect AF with non optimised lenses is not 100% reliable, though I find it often is reliable, though better with some lenses than others.

So far, no Four Thirds body from either Panasonic or Olympus supports continuous focus in live view mode. It's a different story with Micro Four Thirds, where the user has no option but to use Live View and both Panasonic and Olympus have implemented C-AF in live view.

Tomorrow: Differences between Panasonic/Leica and Olympus Zuiko lenses.

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  1. Barr1e's Avatar
    Thanks for putting another part of the jig saw together. I look forward each day to this section of the site. I find it most interesting.

    Regards. Barr1e