Digital Camera Sensors: Size Really Matters

For sensors, size means everything, (sensors) size does matter. 

The larger the sensor, the better the picture and the higher the resolution it can push while maintaining a good image quality. Both compact cameras and SLRs can easily reach 14 megapixels, but the difference in image quality, clarity, and noise at that resolution is remarkable. My iPhone 4s can produce 8MP image, but the quality of the image produced is far behind Nikon D70s. For sensors, the larger the better.

Sensor size can vary even among digital SLRs. These cameras generally use either APS-C or full frame sensors. Full frame is based on 35mm film. A full frame camera's sensor is 35mm wide, the same size as a frame of 35mm film. 
 

These sensor sizes are important in digital SLRs not only for picture quality, but for lens choice. Non-full-frame cameras employ a "crop factor" when using lenses intended for 35mm cameras. APS-C cameras use a crop factor of 1.5 to 1.7, meaning a lens that's 50mm on a 35mm camera works the same as a 75 to 85mm-equivalent lens on an APS-C camera. Full frame SLRs, like Nikon's D3x and  Canon's 5D don't have a crop factor; a 50mm lens acts like a 50mm lens.  

Micro Four Thirds cameras sit between compacts and SLRs, with sensors measuring approximately 18 x 13.5mm. The sensors are still much larger than compact cameras, but don't even reach APS-C size.. 

On the other end, some professional photographers use much larger and more expensive cameras than even full frame SLRs. Medium format cameras, like those made by Hasselblad, use sensors much larger than full frame.

Crop Factor

The crop factor is the sensor's diagonal size compared to a full-frame 35 mm sensor. It is called this because when using a 35 mm lens, such a sensor effectively crops out this much of the image at its exterior (due to its limited size). the focal length multiplier relates the focal length of a lens used on a smaller format to a 35 mm lens producing an equivalent angle of view, and is equal to the crop factor. This means that a 50 mm lens used on a sensor with a 1.6X crop factor would produce the same field of view as a 1.6 x 50 = 80 mm lens on a 35 mm full frame sensor.

The lens focal length does not change just because a lens is used on a different sized sensor — just its angle of view. A 50 mm lens is always a 50 mm lens, regardless of the sensor type. At the same time, "crop factor" may not be appropriate to describe very small sensors because the image is not necessarily cropped out (when using lenses designed for that sensor).

 Depth of Field

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. 

A shallower depth of field may be desirable for portraits because it improves background blur, whereas a larger depth of field is desirable for landscape photography. This is why compact cameras struggle to produce significant background blur in portraits, while large format cameras struggle to produce adequate depth of field in landscapes.

Pixel Size, Noise Level, and Dynamic Range

Larger sensors generally also have larger pixels (although this is not always the case), which give them the potential to produce lower image noise and have a higher dynamic range. Dynamic range describes the range of tones which a sensor can capture below when a pixel becomes completely white, but yet above when texture is indiscernible from background noise (near black). 

Further, larger pixels receive a greater flux of photons during a given exposure time (at the same f-stop), so their light signal is much stronger. For a given amount of background noise, this produces a higher signal to noise ratio — and thus a smoother looking photo.