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Tuesday, April 22, 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
This rendition of a Fujifilm X-Trans II CMOS sensor shows the key components: microlenses, Bayer array and photosites. The diagram also shows how on-sensor phase-detection AF works.



The PhaseOne IQ250 (top) and Hasselblad H5D-50c (above) have 44x33mm CMOS sensors, marking a shift from CCD.
CMOS And CCD
Digital cameras—smartphone through pro medium-format—use one of two types of image sensors: CMOS (complementary metal-oxide semiconductor) or CCD (charge-coupled device). Early digital cameras used CCD because it produced better image quality and was easier to make using the technology of the time and, basically, was the only type available at the time. Now, interestingly, CCD is found mainly in compact digital cameras—and high-end medium-format digital cameras and backs. Canon has been producing its own CMOS sensors for DSLRs since 2000; other DSLR makers joined in en masse when Sony introduced its Exmor CMOS sensor in 2007.

Both types of sensors do essentially the same thing: convert light (photons) into an electric charge (electrons) and then voltage, and then convert that into numbers (digital 1s and 0s). Yes, digital image sensors are analog devices—the output isn't digital until the A/D (analo g-to-digital) converter makes it so. CCDs pass the charge from pixel to pixel and convert it to voltage as it leaves the chip. CMOS sensors convert the charge to voltage in each pixel, and some even do A/D conversion on-chip.

CMOS sensors use less power and are less costly to produce, and image quality has caught up with CCD, so today, many higher-end compact digital cameras and all DSLRs use CMOS. In DxOMark.com's sensor testing, 11 "full-frame" (36x24mm) CMOS sensors have scored in the ballpark with the best of the medium-format CCD sensors tested, and the even smaller 19-megapixel RED EPIC DRAGON CMOS sensor recently became the first to top 100 on the DxO scale.

CMOS is faster than CCD, making it easier to produce HD video (although its rolling shutter makes it more prone to wobbly effects when the camera is panned during a video) and enabling such features as high-speed still shooting, in-camera HDR, contrast-based AF off the image sensor and 1080p full HD video.


Cutaway of a sensor showing a typical Bayer array.
CCD delivers a "look" favored by many medium-format shooters, based on color reproduction, dynamic range and tonal smoothness, but even medium-format has started going CMOS. While CCD has done wonderfully well for medium-format shooters in terms of low-ISO image quality, it has lacked high-ISO performance, shooting speed and DSLR-style Live View on the LCD monitor. Now, Phase One (IQ250) and Hasselblad (H5D-50c) have introduced models with a 44x33mm CMOS sensor from Sony (with much input from the camera makers) that solves these CCD drawbacks, while delivering the image quality medium-format shooters have come to expect. The new CMOS sensor even delivers one stop more dynamic range than Phase One's flagship IQ280/IQ260 CCD cameras. Rumor has it that Pentax is also working on a camera using this sensor.

To Bayer, Or Not To Bayer
From the start, the vast majority of digital cameras, whether CCD or CMOS, have used Bayer sensors, which feature a grid of red, green and blue filters, one over each pixel site. The photodiode ("pixel") receives light of only one primary color. That's because the photodiodes are colorblind; they detect how much light is striking them, but not what color (wavelength) it is. The Bayer filter grid (named after the Kodak scientist who devised it) allows these colorblind sensors to deliver full-color images.

 

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