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Tuesday, April 29, 2014

Sensors Un-Sensored

The heart of any digital capture system is the image sensor. Incredibly fast-paced improvements in resolution and performance have brought us to a point where the next quantum leap is on the horizon.

This Article Features Photo Zoom
Fujifilm has tried a number of different filter patterns, currently offering in their X-Trans sensors an RGB filter grid that arranges the red, green and blue filters in a more random array, with all three colors in each horizontal and vertical pixel row. This minimizes moiré and false colors, allowing Fujifilm to do away with the sharpness-robbing optical low-pass filter required by most Bayer-sensor cameras. The newest X-Trans II CMOS sensor incorporates more than 100,000 phase-detection pixels for quicker AF in good light.

On-Sensor Phase-Detection Autofocus
This brings us to another sensor trend: putting phase-detection AF (PDAF) sensors on the image sensor. Theoretically, PDAF can tell from a single reading whether or not the image is in focus, and if not, which way it's off and by how far. It can also calculate from successive readings how fast and in what direction the subject is moving, and predict where it will be at the instant of exposure. All DSLRs feature phase-detection AF when you're using the eye-level SLR viewfinder.

Most cameras, including DSLRs, use contrast-based AF in Live View mode because the SLR mirror has to be in the up position for light to reach the sensor, and you can't see through the optical finder when the mirror is up. Contrast AF reads image contrast right off the image sensor, so it's more accurate than PDAF, but it's also slower. The system has to take a contrast reading, move focus one way or the other, and then take another reading. If the contrast is better at the new position, the system adjusts focus another step in that direction, repeating until contrast drops off. Then it goes back to the step with the highest contrast. If the second reading shows less contrast than the first, the system moves the lens the other direction and repeats. New contrast-based AF systems are very quick, some taking 120 measurements per second, but they still have to take more measurements than PDAF to determine focus, and they can't easily predict a moving subject's position at the instant of exposure.

These Canon diagrams demonstrate how on-sensor phase-detect AF works. The system has the potential to be faster, smoother and more accurate than previous AF systems, and having AF on the sensor like this doesn't reduce the effective resolution.

There's a trend to put some PDAF sensors on the image sensor so they can work during live-view operation. These hybrid systems range from a handful of PDAF sensors to Canon's Dual Pixel CMOS AF in the 20.2-megapixel EOS 70D, which actually has two photodiodes at each pixel site—during AF, the dual photodiodes provide PDAF, and during image capture, they combine to output the image signal as single pixels. The 70D has a separate 19-point AF system for viewfinder (non-live-view) shooting.

Sony's SLT cameras feature a unique translucent SLR mirror that doesn't move as conventional SLR mirrors do during shooting. Instead, the SLT mirror transmits most of the light from the lens to the image sensor, while simultaneously reflecting a portion up to the PDAF sensor assembly so you get full-time PDAF during live view (the SLTs are always in Live View mode). The full-frame SLT-A99 combines this with 102 PDAF sensors overlying the image sensor to further improve performance on moving subjects.

Future Developments
As sensor technology evolves, dramatic advancements are on the horizon. Innovation doesn't happen on a precise timetable, but we're keeping an eye on a few imminent developments. High-end still plus motion shooting is currently the domain of RED. The Digital Still & Motion Camera concept is a fundamental part of RED's DNA. Other manufacturers are catching up, and we expect to see this capability—being able to shoot full motion where each frame is also a full-resolution still image—in more cameras in the future. We're also particularly intrigued about the future of high-speed motion cameras. The recent Edgertronic Kickstarter campaign is pointing to the future of affordable high-speed motion capture; the cameras are less than $6,000. Again, as still capture and motion capture continue to merge, high-speed technology will be a key aspect of true "hybrid" shooting.


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