Sensors come in many different “formats” of dimension and aspect ratio. One of the most common sizes, used in compact cameras is 1:1.7", whose dimensions are approximately 7.5mm x 5.6mm. It is said to have an aspect ratio of 4:3; that is its width is four-thirds of its height (or expressed another way, it height is three-quarters of its width). Another popular size is the APS-C format, approximately 23.6mm x 15.7mm, derived from the film format of the same designation. “Full-frame” sensors are the same dimensions and aspect ratio of 135 film. The Micro Four-Thirds format is the most recent format, employing a 4:3 aspect ratio in a sensor that is half the physical width and height of a full-frame sensor.
Unlike film, which has a theoretical limit to resolution, sensor size does not determine resolution, although it does determine image quality. Both a 1/1.7" and a full-frame sensor (which is 480% larger) can produce a 12 megapixel image, but the full frame image will be sharper and less noisy.
Like film, sensors fall into different classes, those 36mm x 24mm and under being small format, and those larger being medium format, such as the Kodak KAF3900 at 50.7mm x 39mm with an aspect ratio closer to 4x5 sheet film. Large format sensors may be of the scanning-back variety, which move a narrow sensor across the width of the frame over time. These however are only suitable for long exposures.
There are two types of sensor technologies currently in use; CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor). CCD sensors are typically found on compact cameras due to their slower speed and smaller size. CMOS sensor are employed on larger cameras due to their faster speed, made possible by incorporating some image processing operations directly on the chip. This in turn is made possible by their larger physical size.
Sensors have different color and sensitivity characteristics just like film, but as they are a permanent part of the camera, you cannot select among them as they are not interchangeable. Instead the imaging engine of the camera which “processes” the raw image information from the sensor governs how the image is captured and stored in terms of color balance and sensitivity to light.
Like film, digital cameras can be set to emulate different ISO characteristics. A high ISO setting will permit faster shutter speeds and lower light capability at the expense of image quality. The degradation in image quality is known as “noise” a randomization of the pixel structure similar to film grain. A lower ISO will produce higher quality images at the expense of slower shutter speeds and brighter lighting requirements.
Although the light-collecting areas of the sensor, known as photosites, can be made very small, they cannot be made as thin as film. Therefore, light rays that do not strike the sensor perpendicular to its surface do not resolve as accurately as those that do. This is particularly true of wide angle lenses. To address this issue, a system of tiny lenses is placed over the sensor to help direct the light rays. Lenses optimized for digital photography employ telecentric design, which better control the angle of the light rays exiting the rear of the lens and thus improve results.
Sensor Sizes
Medium Format
crop factor 0.66x, aspect ratio 4:3, area 1,728mm²
Used in the Mamiya RZ33 (33MP) medium format DSLR
Kodak KAF-37500 (Leica) (45mm x 30mm)crop factor 0.80x, aspect ratio 3:2, area 1,350mm²
Used in the Leica S2 (37.5MP) medium format DSLR.
Small Format
35mm Full Frame (36mm x 24mm)crop factor 1.0x, aspect ratio 3:2, area 864mm²
Used in 135 film cameras, Canon EOS 5D Mark II (21.1MP) ; Leica M9 (18MP), Nikon D3X (24.5MP), D700 (12MP), Sony a900 (24.6MP).
Also Known as Nikon FX
crop factor 1.26, aspect ratio 3:2, area 513mm²
Used in older Canon DSLRs
crop factor 1.33x, aspect ratio 3:2, area mm²
Used in Leica M8.2 (10.3MP)
APS-C (Nikon) (23.6mm x 15.7mm)crop factor 1.5x, aspect ratio: 3:2, area 370.5mm²
Used in APS film cameras; Fujifilm Finepix X100 (12.3MP); Nikon D300S (12MP), D7000 (16.2MP), D90 (12MP), D5100 (16.2MP), D3100 (14.2); Pentax K5 (16.3MP), Kr (12.4MP); Samsung NX200 (20.3MP); Sony a580 (16.2); NEX-7 (24.3MP)
Also known as Nikon DX
crop factor 1.6x aspect ratio 3:2, area 328.6mm²
Used in Canon DSLRs
Premium
Four Thirds (17.3mm x 13mm)crop factor 2.0x, aspect ratio 4:3, area 224.9mm²
Used in Four Thirds and Micro Four Thirds cameras; Olympus DSLRs, PEN Series.
crop factor 2.7x, aspect ratio 3:2, area 116.2mm²
Used in Nikon 1 V1, J1
crop factor 2.5x, aspect ratio 4:3, area 122.9mm²
Used in commercial video surveillance cameras.
2/3" (8.8mm x 6.6mm)crop factor 3.6x, aspect ratio 4:3, area 58.1mm²
Used in Fujifilm Finepix X10 (12MP).
1/1.7" (7.6mm x 5.7mm)crop factor 4.21x, aspect ratio: 4:3, area 43.3mm²
Used in “bridge” and “premium” mid-sized cameras; Canon PowerShot G12 (10.1MP), PowerShot S95 (10.1MP); Nikon Coolpix P7100 (10.1MP), Coolpix P5100 (12.1MP).
Compact
1/2.3" (6.16mm x 4.62)crop factor 5.19x, aspect ratio: 4:3, area 28.5mm²
Used in compact “shirt pocket” cameras, Canon PowerShot SD780 IS (12.1MP), Nikon Coolpix S series; Pentax Q, Optio, RZ series.
1/2.5" (5.76mm x 4.29mm)crop factor 5.59x, aspect ratio 4:3, area 24.7mm²
Used in compact “shirt pocket” cameras.
crop factor 7x, aspect ratio 4:3, area 15.3mm²
Used in the iPhone 4S (8MP)
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