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What's the Difference Between CMOS, BSI CMOS, and Stacked CMOS?

Most modern digital cameras use CMOS tech to capture light, but not all are equally capable. We break down the various types of CMOS sensors and differentiate them from CCD and Foveon chips.

(Credit: Jim Fisher)

The heart of a digital camera is its image sensor, and in the digital age, we've seen a few different types of sensor technologies evolve. Most current options use a version of Complementary Metal-Oxide-Semiconductor (CMOS) technology. These CMOS chips have some real advantages over the Charged Coupled Device (CCD) sensors common in the early days of digital photography, including improved power efficiency and heat control. Those improvements paved the way for 4K video (and beyond) in swappable lens cameras.

But there's more than one type of CMOS sensor. And if you're shopping for a new mirrorless camera, you might be overwhelmed by different architectures and unsure why Stacked CMOS and global shutter cameras cost so much more than baseline models. Read on for some insight into how the various flavors of CMOS differ.


Architectural Differences

For the most part, digital sensors are built around a similar concept, even if there are differences in the chip construction. The imager features light-sensitive photosites and a filter with repeating patterns of red, green, and blue squares adding color. Most sensors use a 4-by-4 Color Filter Array (CFA) called a Bayer CFA (named after its creator), but some Fujifilm models use a more complex 6-by-6 X-Trans CFA.

The Bayer CFA (left) has a simple four-by-four repeating pattern, while the Fujifilm X-Trans (right) uses a six-by-six grid to capture photos in color
(Credit: Bob Al-Greene)

You might also run into Quad Bayer sensors, a type common in smartphones with huge pixel counts, as well as action cams and drones. These sensors pack high pixel counts—48MP is a common figure—but output lower-resolution images via a technique called pixel binning. It's a feature that's making its way into interchangeable lens camera (ILC) sensors in slightly different ways, too. The luxury-priced Leica M11, for instance, leans on pixel binning to snap photos at resolutions of 60MP, 36MP, or 18MP.

Pixel Binning combines smaller Quad Bayer pixels into larger groups
(Credit: Bob Al-Greene)

CMOS chips differ from earlier-generation CCDs in a few important ways. For instance, CMOS chips read out data pixel-by-pixel in what's called a rolling electronic shutter, instead of all at once like a CCD. But there are technical advantages that made the photo industry move away from CCD. For instance, CMOS chips put their analog-to-digital converter (ADC) on board, instead of in a separate unit. This helps the chips draw less power and generate less heat than CCDs, good for both low-light image quality and battery life.


CMOS, BSI CMOS, Stacked CMOS, and Global Shutter

There are four main types of CMOS sensors. The basic CMOS design lives on today in entry-level and midrange cameras—models that get the latest features a couple of generations after they appear in high-end models.

The improved design, Backside Illuminated (BSI) CMOS, is similar in concept to regular CMOS, but these chips arrange the components differently. In short, the photosites are further forward on the die and the line-by-line readout speed is brisker. This change enables practical advantages: Generally speaking, a BSI CMOS is around an f-stop better when it comes to image noise. That means a BSI CMOS shows as much noise at ISO 12800 as a similar CMOS chip would at ISO 6400. It also means that APS-C and Micro Four Thirds cameras with BSI chips play on more even footing with full-frame CMOS cameras. Those aren't hard-and-fast rules, but they're good guidelines to follow.

This diagram shows the architectural differences between CMOS, BSI CMOS, and Stacked CMOS sensors
(Credit: Bob Al-Greene)

The quicker readout speed makes a fully electronic shutter possible for BSI CMOS models, plus allows for faster autofocus response for higher burst rates with autofocus. The Fujifilm X-T3 was the first consumer camera to really take advantage of these features. It debuted with 20fps focus with a fully electronic shutter in 2018. You still need to use a mechanical shutter to reliably freeze subjects in motion with most BSI CMOS cameras, but the silent electronic shutter comes in handy for portraiture and other still subjects.

Stacked CMOS chips improve upon the BSI CMOS concept. They place components in a similar arrangement, but the design also stacks the image signal processor and its ultra-fast DRAM memory into the same silicon. This makes readout speeds even faster. The first mainstream Stacked CMOS camera, the Sony a9 from 2019, made waves by offering an interruption-free photographic experience—you can use it to fire off photos at 20fps without losing view of your scene.

The Nikon Z 9's Stacked CMOS sensor reads out fast enough to eliminate the need for a mechanical shutter, a true feat for a camera made to freeze moving subjects in place
(Credit: Jim Fisher)

Because the tech makes this type of photography possible, Stacked CMOS chips have become the de facto standard for the high-end ILCs pros use to photograph from the sidelines or a press box. We've seen some cameras hit 30fps (the Sony a1 and Canon EOS R3), and the Nikon Z 9 manages 11MP photos at 120fps because of its stacked chip and dual processors. Extra-fast readout and processing power also make for better autofocus. Stacked chips now exceed BSI CMOS sensors in focus speed, accuracy, and subject recognition. This all works together to ensure that stacked cameras don't just take a bunch of photos in a row; they take a bunch of in-focus photos in a row.

A global shutter CMOS sensor takes things one step further. It doesn't just scan across the sensor quickly, it does it in an instant. We've tested one consumer camera with this tech to date, the Sony a9 III. Its implementation supports 120fps tracking at full resolution, flash sync at previously unheard-of 1/80,000-second speeds, and the ability to freeze a slice of time perfectly still. The speed comes at a cost, however. Noise and dynamic range are more on the level of classic CMOS chips, but action photographers might see reason to trade away some image quality for speed.

In summary, CMOS chips are the mainstream, basic options for today's digital cameras. Stepping up to a model with a BSI CMOS sensor ups readout speed and improves low-light imaging. Stacked CMOS chips push the speed envelope even further and keep your subject in perfect view as the camera makes an image. Finally, you need to turn to a global shutter if you want to perfectly freeze motion in time and don't mind giving up a bit of image quality.


CCD, Foveon, Monochrome, and Full Spectrum Cameras

We touched on CCD sensors earlier. These chips were the standard for consumer cameras in the aughts but gave way to CMOS in more recent years. The former still has proponents but, aside from low-end compacts, you don't see the sensor in modern consumer models.

The Sigma DP2 Merrill used to take this photo captures colors with a three-layer Foveon sensor, a different approach than Bayer cameras take
(Credit: Jim Fisher)

Foveon is another sensor type that the Sigma X3, Merrill, and Quattro cameras use exclusively. Foveon chips record color differently via a trio of light-sensitive layers instead of a color filter array. On the plus side, these cameras don't need to interpolate to fill in missing color, which means they can capture much more detail than a similar pixel-count Bayer sensor. But there are drawbacks, too: Raw processing apps don't support files from many Foveon cameras and photos show heavy noise at moderate ISOs. There's only one Foveon model on the market today, the Sigma dp Quattro. We missed reviewing the camera but did look at the now-discontinued Quattro H, which has many of the same features and a Foveon sensor that's a little larger than APS-C.

Specialty cameras like the Leica M10 Monochrom omit a color filter array to capture the world in black and white; I took this in-the-mirror photo with the M10M and a post-war Elmar 5cm F3.5 lens
(Credit: Jim Fisher)

Monochrome cameras are another variation. Leica makes a few specialty options that ditch the color filter array and capture black-and-white images exclusively. The M10 Monochrom and Q2 Monochrom are excruciatingly expensive but monochrome specialists may find them to be worth the cost. These cameras show an advantage in detail, much like Foveon chips, but are superior to color options at high ISOs—cutting out the Bayer filter just about doubles the amount of light that hits the sensor.

You won't find any Full Spectrum Infrared cameras on sale at your local big-box store, but they exist. Consumer cameras have a filter over the sensor to cut out invisible light. But companies such as KolariVision and MaxMax can remove this filter or sell you a pre-converted camera that can see infrared and ultraviolet wavelengths. Landscape specialists love using these sensors to snap surreal, alien-looking scenes right here on Earth.

Cameras converted for infrared, ultraviolet, or full spectrum imaging capture wavelengths of light that your eyes can’t see
(Credit: Getty / Justin Reznick Photography)

Picking the Right Camera

Now that you know more about sensors, it's time to pick up a camera. If you're shopping for a swappable lens model, check out our picks for the best mirrorless and best full-frame cameras, or read our more general buying guide if you're not sure what type of camera you should purchase.

About Jim Fisher