The image sensor in a digital camera takes the place of the film plane, and collects light that is converted into an electronic signal. Sensor size is the most important aspect of digital imaging, and far more influential in determining the quality of a given image than other factors, namely the number of megapixels. When considering the sensor size of a camera, understand that size definitely matters here– the larger the size of the sensor, the greater the ability to achieve very fine detail in the image, and thus, larger printing sizes and an ability to crop the image down and still retain detail.
Dot pitch, also known as pixel pitch, is a measurement of the distance between pixels on the image sensor, therefor an indicator of the size of these pixels. A larger sensor has a larger dot pitch, which means larger pixels, and in turn enables the sensor to collect more light and capture an image of relatively higher resolution. This is especially the case in lower lit scenes where a higher ISO would be used. Using a higher ISO on a camera with a large image sensor produces less noise than using a higher ISO on a camera with a smaller image sensor.
Some of the most common sensor sizes (smallest to largest), and examples of their typical camera bodies:
1/2.3″ Mainstream compact cameras. These sensors are very limited in capturing light in low lit scenes without flash because of their small sensor size (and also smaller pixels on the image sensor.
1/1.7″ Most typical of higher end compacts, e.g. Nikon Coolpix P340, Canon PowerShot S95, Pentax Q7 (entry level mirrorless interchangeable- lens camera) (MILC).
2/3″ Olympus C-8080 Wide Zoom (Ultra zoom fixed lens compact, released 2004), Nokia Lumia 1020 (current leader in resolution for cell phones)
1″ Sony RX100, FujiFilm X-S1, Nikon 1 Series cameras. These cameras tend to bridge the gap between compact cameras and the larger and more expensive APS-C and Four-Thirds cameras.
4/3″ A relatively new camera system introduced by Olympus and Panasonic in 2008. Allows for compatibility of lenses and camera bodies between different manufacturers.
Fovean (Now owned by Sigma) A different sensor design that collects data via photosites in separate stacked sensors for each primary color wavelength (red, green, and blue). Although this sensor size is unique to Fovean type sensors, Sigma’s latest DSLRs employ a APS-C sized sensor (23.5 x 15.7)
APS-C (green) Canon EF-S format DSLRs- such as the 7D, and Rebel Series
APS-C (light blue) Nikon DX format DSLRs (e.g. D7100, D5300), Sony NEX-7
APS-H Sony Alpha 7S, Leica M8, Canon EOS-1D Mark IV (released Oct. 2009). An older sensor size that will likely not be seen again, at least in main stream DSLRs.
Full Frame Most prosumer and all pro level DSLRs: e.g. Nikon D610, Nikon D800 (FX format), Canon 5d Mk III (EF format), Sony a99 (Alpha Series), Leica M9 (digital rangefinder camera). As these sensors are identical in size to a piece of 35 mm film they are referenced as “full frame”. (There is still debate about the use of this term in the photography world.)
Sensors have come a long way since the advent of DSLRs right before the dawn of the 21st century. They have also become more affordable to produce for maunufacturers, as it usually goes with technology these days. This occured with the introduction of complementary metal-oxide semiconductor (CMOS) sensors, which are indeed less costly to manufacture and also use less power than earlier charge-coupled device (CCD) type sensors. While both types perform the same function of converting light into electrons, CMOS sensors are used more often these days (Nikon has not put a CCD type sensor in a new camera since the D3000 was announced in the fall of 2009). For the sake of comparison, the first Nikon DSLR was the Nikon D1 (announced June 1999) with a 2.74 MegaPixel CCD type sensor (23.7 x 15.6mm), and retailed for about $5,000.
It used to be that the larger the sensor (or film size), the larger the camera body, and also the larger the lens needed to place the larger image onto the sensor. This is starting to change with the advent of mirrorless cameras, and now even compact cameras sporting full-frame sensors— such as Sony’s fairly new Cyber-Shot RX-1.
At 36 mp, the Nikon D810 currently offers the highest resolution available (at least for DSLRs), although there are rumors of Canon bringing out a high resolution series of the EOS later this year.