Stacked CMOS

CMOS, BSI CMOS, And Stacked CMOS Differences

Stacked CMOS

Not all current digital cameras use CMOS to capture light. We compare CMOS, CCD, and Foveon sensors. In the digital age, several image sensor technologies have emerged. Most alternatives employ CMOS technology. CMOS chips have improved power efficiency and thermal control over CCD sensors used in early digital photography. These developments enabled 4K video in interchangeable lens cameras.

There are multiple CMOS sensors. If you’re shopping for a new mirrorless camera, you may be confused by the different architectures and why Stacked CMOS models cost more. Learn how CMOS tastes differ.

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Differing architectures

Digital sensors have a similar idea, despite variations in chip design. The imager has light-sensitive photosites and a red, green, and blue filter. Most sensors utilise a 4-by-4 Bayer CFA, however certain Fujifilm models use a 6-by-6 X-Trans CFA.
Quad Bayer sensors are ubiquitous in high-pixel smartphones, action cams, and drones. These sensors have large pixel counts—48MP is common—but output lower-resolution images via pixel binning. This feature is also appearing in interchangeable lens camera (ILC) sensors. Leica M11 uses pixel binning to take 60MP, 36MP, or 18MP photographs.
CMOS chips are different from older CCDs. CMOS semiconductors read data pixel-by-pixel in a rolling electronic shutter, unlike CCDs. The picture business moved away from CCD for technical reasons: CMOS chips have their analog-to-digital converter (ADC) on board. The chips drain less power and heat than CCDs, improving low-light image quality and battery life.

Stacked, BSI, and CMOS

Three CMOS sensor types exist. The core CMOS design lives on in entry-level and midrange cameras, which acquire new features after high-end versions.

Backside Illuminated (BSI) CMOS is comparable to conventional CMOS but arranges components differently. Photosites are further forward on the die, and line-by-line readout is faster. BSI CMOS reduces image noise by around an f-stop. A BSI CMOS chip has the same amount of noise at ISO 12800 as at ISO 6400. APS-C and Micro Four Thirds cameras with BSI chips can compete with full-frame CMOS cameras. These are guidelines, not hard-and-fast regulations.

The increased reading speed allows BSI CMOS models to have fully electronic shutters and faster autofocus for larger burst rates. First consumer camera to use these features was Fujifilm X-T3. It debuted in 2018 with a 20fps electronic shutter. Most BSI CMOS cameras still require a mechanical shutter to freeze moving scenes, but the electronic shutter is useful for portraits and other still pictures.

Stackable CMOS chips improve on BSI CMOS. The image signal processor and its ultra-fast DRAM memory are stacked on the same silicon. This accelerates readouts. The first mainstream Stacked CMOS camera, the Sony a9 from 2019, made headlines by giving an interruption-free photographic experience.
Stacked CMOS chips have become the standard for high-end ILCs used for sideline and press box photography. The Sony a1 can shoot 30fps, and the Nikon Z 9 can shoot 11MP photographs at 120fps thanks to a stacked chip and dual processors. Faster readout and processing improve autofocus. Stacked chips are faster, more accurate, and better at subject recognition than BSI CMOS sensors. This ensures that stacked cameras take in-focus photographs in a row.
CMOS chips are standard in digital cameras today. Readout speed and low-light imaging improve with a BSI CMOS sensor. Stacked CMOS chips boost speed and maintain your subject in view when the camera takes a picture.

Full-spectrum, CCD, Foveon, and monochrome cameras

CCD sensors were mentioned. CMOS has replaced these semiconductors as the standard for consumer cameras. The former has proponents, but you won’t find it in modern consumer cameras.
Sigma X3, Merrill, and Quattro utilise Foveon sensors. Instead of a colour filter array, Foveon chips use three light-sensitive layers to record colour. These cameras don’t interpolate missing colour, therefore they can record more detail than a similar-pixel-count Bayer sensor. There are downsides: Raw processing software don’t support many Foveon cameras, and photographs at modest ISOs are noisy. Only the Sigma dp Quattro Foveon model is available. We missed evaluating the camera, but did look at the now-discontinued Quattro H, a sister product with similar capabilities but a larger Foveon sensor.
Variations include monochrome cameras. Leica makes black-and-white-only cameras that lack colour filters. Monochrome enthusiasts may find the M10 Monochrom and Q2 Monochrom worthwhile. Cutting off the Bayer filter doubles the amount of light that hits the sensor, making these cameras superior to colour choices at high ISOs.
Full Spectrum isn’t available. Big-box stores sell infrared cameras. Consumer cameras filter out invisible light. KolariVision and MaxMax can remove this filter or sell you a pre-converted infrared and UV camera. Landscapers use these sensors to capture alien-like scenes on Earth.
Sony’s Semiconductor Solutions Group is creating a next-generation Stacked CMOS chip that repositions onboard transistors for higher dynamic range and lower ISO noise. The development was announced in late 2021, but we expect to wait a few years for a consumer camera.

Panasonic’s developing a new sensor. The company’s Organic sensor uses an Organic Photoconductive Film (OPF) layer instead of separate pixels to capture light.

Camera Selection

After learning about sensors, get a camera. Check out our suggestions for the best mirrorless and full-frame cameras if you’re shopping for a swappable lens model.

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