The soul of a digital camera is its sensor--to determine image size, resolution, low light performance, depth of field, dynamic range, lenses, and even the camera's physical size, the sensor is key.

An image sensor is a solid state device, the part of the camera's hardware that captures light and converts what you see through a viewfinder or LCD monitor into an image. Think of the sensor as the electronic equivalent of film. With film cameras, you could choose from hundreds of film brands, each with it own unique and identifiable characteristics. With digital cameras, much of that technology is built into the hardware, and special film-like effects are applied later with software.

Your camera's sensor determines how good your images look and how large they can be scaled or printed. Image quality depends on not only the size of the sensor, but also how many millions of pixels (light sensitive photosites) fit on it, and the size of those pixels.

The sensor size also affects what you see through the viewfinder--the relationship between what you're shooting and what actually gets recorded in the frame and passed through to your memory card. Smaller sensors apply a crop factor to lenses, capturing less of the scene than full frame sensors. The full frame reference point is always traditional 35mm film.

Confused yet? Don't be. Even if you don't know a CCD from a CMOS from a Four-Thirds from an APS-C, this guide breaks down that intimidating alphabet soup and walks you through the sensors you're likely to encounter.

Sensor types

The most common types of sensors are CCD (charged coupled device) and CMOS (complementary metal--oxide--semiconductor).

CCD is one of the oldest image capture technologies for digital cameras and has long offered superior image quality compared to CMOS sensors, with better dynamic range and noise control. While still prevalent in budget compact models, the CCD's basic construction and greater power consumption has caused it to be largely replaced by CMOS alternatives.

CMOS has been considered an inferior competitor to CCD, but today's CMOS sensors have been upgraded to match and even transcend the CCD standard. With more built-in functionality than CCDs, CMOS sensors work more efficiently, require less power, and perform better for high-speed burst modes.

The newer Foveon X3 sensor, based on CMOS technology, is used only in Sigma's compact cameras and DSLRs. Live MOS is a brand name for image sensors used by Leica, Panasonic, and Olympus in their Four Thirds System DSLRs manufactured since 2006. They reportedly offer CCD image quality with the lower power consumption of a CMOS.

Sensor sizes

Full Frame (36 x 24mm):The largest sensor size is called full frame, the same as a frame of 35mm film. Full frame sensors are almost twice as big as APS-C sensors, the next size down. Hefty pro-level beasts such as the Nikon D800, the Canon EOS 5D Mark III, and the Nikon D4 have full frame sensors. However, over the last year, smaller fixed lens cameras like the Sony Cyber-shot RX1 also feature full frame sensors. The Sony Alpha SLT-A99, a DSLR-like camera that has a fixed transluscent mirror as opposed to one that flips back to capture a shot, is also a full-frame model.

With full frame sensors, there's no crop factor, so what you see through the viewfinder is what you shoot. Larger sensors in full-frame models, when paired with wide aperture lenses, can deliver extremely shallow depth of field--great for macro and video work. Generally, the larger the sensor, the bigger the camera body and the larger the lenses needed to take advantage of it, though that is starting to change.

APS-H (28.7 x 19mm): APS (active pixel sensor)  is the most popular sensor for both interchangeable lens and higher quality fixed lens cameras, and it is used for a vast portion of consumer and prosumer DSLRs. The APS-H type combines a comparatively large sensor with a moderate pixel count to boost speed and ISO performance. It applies a 1.3x crop factor to mounted lenses. The Canon 1D Mark IV and Canon 1D Mark III feature this kind of sensor.

APS-C (23.6 x 15.8mm): Most major brand prosumer and enthusiast DSLRs from Canon, Nikon, Pentax, and Sony use an APS-C sensor--and not all APS-C sensors are equal. A Canon APS-C sensor measures 22.2 x 14.8mm while the Sony, Pentax, Fujifilm, and Nikon (DX) range from 23.5 x 15.6mm to 23.7 x 15.6mm. The Ricoh GR (23.7 x 15.7mm), the Canon EOS M (22.3 x 14.9mm), the Sony NEX line, and Fujifilm all use APS-C sensors.

Four Thirds (17.3 x 13mm): Roughly a quarter of the size of a full-frame sensor, four thirds is an open DSLR standard created by Olympus and Kodak and is used for all Olympus and Panasonic Four Thirds and Micro Four Thirds DSLRs. It has a 2x crop factor, which doubles the effective focal length of a mounted lens. The new Olympus OM-D E-M1, the Olympus Pen E-PL5, and the Panasonic Lumix GH1 use this sensor.

CX format (1 inch): Announced in 2011, Nikon's CX format has been adopted on the Nikon 1 camera system. Sony also released the pocket-size Cyber-shot DSC RX100 digital camera in 2012 that also uses a 1-inch sensor (13.2 x 8mm) with a 2.7x crop factor.

Small sensors

The last two sizes listed below make no practical sense. As a point of reference, these small sensor definitions are derived from the size of video camera tubes used in CRT TVs circa the 1950s.

1/1.7 inch (7.6 x 5.7mm): Among the largest sensor sizes used in compact cameras, these allow larger pixels for improved noise performance over standard pocket point-and-shoots. Larger pixels more ably handle gaps in brightness to reproduce better image detail.

1/2.5 inch (5.76 x 4.29mm): This is among the smallest sensors, typically used in low-priced point-and-shoot models. While cheaper to manufacture, the smaller pixels on these tiny sensors crank up image noise and reduce dynamic range, resulting in less than superlative images. However, that still may be better than a smartphone, especially for portraits of people.

Among other variations in this category are 1/3.2, 1/2.3, 2/3, 1/3.2, 1/1.2, and 1/1.8. With sensors as tiny as 1/2.7-inch (5.37 x 4.04mm), it's no mystery that smartphones are making serious inroads on compact camera sales.

What goes where?

Compact mirrorless interchangeable lens cameras typically feature a range of sensor sizes. Smaller ones include the 1/2.3-inch (6.17 x 4.55mm) sensor in the Pentax Q, and the 1-inch sensor used in the Nikon 1 Series. The Panasonic Lumix GF5, the Olympus Pen series and the OMD EM-5 have Micro Four Thirds 4/3-inch (17.3 x 13mm) sensors.

Advanced compact cameras with fixed lenses, such as the Fuji X100S (23.6 x 15.8mm) and Nikon Coolpix A (23.6 x 15.7mm) feature an ASP-C sensor. The Fujifilm X20 has a 2/3-inch (8.8 x 6.6mm) sensor while the Canon G1 X has a 1.5-inch sensor (18.7 x 14mm).

Standard point-and-shoot cameras such as the Canon PowerShot SX280 HS and the Samsung Galaxy Camera use 1/2.3-inch sensors (6.17 x 4.55 mm), while better ones like the Nikon P7700 have a larger 1/1.7-inch (7.44 x 5.58 mm) sensor.

Smartphones such as the iPhone 5s and the HTC One have a 1/3-inch image sensor (4.8 x 3.6mm) and fewer pixels. The Nokia Lumia 1020 has a 1/1.5-inch 41-megapixel CMOS sensor.  The Sony Xperia Z1 has a 1/2.3 inch 20.7-megapixel sensor. The Samsung Galaxy S4 Zoom features a 16-megapixel 1/2.3-inch sensor.

Packing megapixels

You may have heard of the "megapixel myth," which states that the more megapixels your camera can pile on its sensor, the better your images will be. This is not true. You don't just want more pixels, but rather the right number of pixels relative to the size of the sensor. And the right number depends on what you intend to do with your photos. For massive cropping or extra large printing, extra resolution is useful. For online sharing or conventional printing, resolution is less important. Tradeoffs include large file size and more processing power to handle high-res photos, versus noise in lower-resolution images.

Smaller sensors that cram pixels together tend to have poorer low-light performance and show noisy artifacts.

What to look for

Whenever possible, as you shop for a new camera, pay attention to what kind of sensor it uses. Because the sensor size and type influences such a vast portion of your camera's output, it will prove a critical factor in size, resolution, operation, and price.

Image sources: Canon, Wikimedia Commons