Friday, February 28, 2014

Autofocus Evolution

By David Schloss Published in Advanced Camera Technique
Autofocus Evolution
Olympus has taken a different approach to integrating phase-detect sensors on the chip and have rolled this out for their new flagship camera, the OM-D E-M1. This camera is based on the Micro Four Thirds sensor and replaces the Four Thirds-based E-5 that was their previous flagship.

Four Thirds lenses have phase-detection focusing capabilities and many pieces of glass are still in circulation. The OM-D E-M1 provides shooters who own Four Thirds lenses (and some newer Olympus MFT glass) a fully phase-detect focus system in predictive autofocus modes thanks to a hybrid imaging/phase-detect sensor.

"By putting phase and contrast on the same sensor, we've been able to vastly improve the autofocus on the E-M1," explains Olympus' Sasserath. "Basically, what we've done is scattered the phase-detect pixels on the sensor, we've scattered the left and right channel, and we're interpolating the surrounding pixels to make the final autofocus speed."

Each of these phase-on-chip systems has the same limitation to their practical use in low light. With so few phase-detect pixels (relative to a dedicated phase-detect sensor in an SLR), the use of the phase detect stops being practical when ambient light is low. That means a scene that in the afternoon could use phase-detection focus might use contrast detect when evening starts to fall.

Since contrast detect doesn't perform predictive focus, the light level plays a big part in whether it will be possible to capture an image.

That would have resulted in a similar effect with hybrid-sensor cameras as with SLR cameras; phase detect is active when there's a lot of light, and contrast detection, when the light levels drop.

But the biggest advance in the nascent hybrid chip world is inside the new Canon EOS 70D, which removes this last limitation with a new and exclusive-to-Canon (for the moment anyhow) technology. Every pixel on the camera's sensor is split into two photoreceptors, with one facing right and one facing left.

This allows for a phase-detection system that functions across the full frame on every single pixel. The whole sensor is used to both capture an image and to evaluate phase-detection autofocus. It works in low light and is active in both Live View mode and while capturing video.
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