Fujifilm’s detection system

With the announcement of the Fujifilm F300 EXR and Z800 EXR coming on a day that also saw four other cameras being launched, it would be easy to overlook their most radical feature. Because, with the latest version of its EXR sensor, Fujifilm has achieved something that’s been hoped for but not previously brought to market – phase detection autofocus in a compact camera. The company is claiming the system enables focus times as fast as 0.158 sec. We got some more details of the system from Hitoshi Yamashita, manager of the company’s Technical Support Group.
What’s so special about Phase Detection AF?

All current compact and mirrorless cameras use contrast detection AF, where the lens is racked back and forwards until the camera finds the position that gives the greatest contrast (which signifies being in focus). Their lenses tend to be designed with very light focusing elements that are fast to move so that this process of trial-and-error can be conducted as quickly as possible. Intelligent processing also attempts to minimize the need to hunt too far for focus but there is still some hunting to be done.

Phase detection, the primary focus method used in most DSLRs tends to be faster but works very differently. To understand why what Fujifilm is trying to do is so desirable, it’s first necessary to understand how it works.Although he wouldn’t be drawn on the exact arrangement he does give some detail: ‘The AF sensels are only arranged in the center area of a CCD, so when phase detection AF is activated the AF point is fixed to the center of an image.’

Of course this means some sensels are receiving 50% less light than their neighbours. Yamashita suggests it need not be a big problem: ‘we use several tens of thousands of pixels in the center area of a CCD, which is a very small number of pixels compared to the 12 megapixels used for imaging.’ And, he says, they don’t simply go to waste when taking pictures: ‘sometimes they are used to compose image data and sometimes not, depending on the situation.’

In low light where there isn’t enough light to allow phase detection (since the focus sensels are only receiving half the available light) or when face detection is needed, requiring the camera to focus away from the center point, the camera switches back to contrast detection AF. As such the company is billing the system as ‘Hybrid AF,’ but that shouldn’t disguise what is a potentially significant development for compact cameras. We’ll be interested to see how the promise of this system works when we get a chance to subject it to testing.

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Tags: detection, system

Category: Fujifilm