Sensor size -
©2013, Terry Mercer.com
Something MOST PEOPLE never think about, or even have awareness of, when making purchasing decisions.
Many long time 'photographers' don't even really understand all the ways sensor size can effect a photograph (positive or negative)... or fail to give it the necessary importance. On a computer screen, with little to no cropping, or other enlarging, the average person will be hard pressed to tell the difference between sensors when comparing photographs taken under optimum lighting conditions; UNLESS the same 'shot' is taken from the same tripod (direction, location, and subject) with the same mm of lens, and even then only when displayed side by side (or stacked on top of each other).
However, there are two other instances when an average person can tell: a) when photo is cropped a great deal, thus drastically enlarged from the original; and, b) when the ISO is set too high for a given camera's true ability to effectively capture an image using available light. These reasons are why a 10 megapixel Professional camera (either full frame sensor or slight crop frame) is usually better than a 10 megapixel consumer camera, which is most always better than a 10 megapixel camera phone.
The difference is the concentration of pixels. Opposite of common sense, smaller pixels on a smaller sensor does NOT create a finer photograph, because they have less space to capture the necessary information. In reality, bigger 'photosites' on a sensor gather more light, produce less-noisy images (i.e., less pixelation), capture a greater dynamic range of colors & light, and perform much better at high ISO settings.
HOWEVER, ultimately HOW YOU SAVE THE FILES is just as important, if not more so... because if you have a compressed sensor, then further compressing the file by saving to JPG rather than TIFF or RAW, you are experiencing compression upon compression... and loosing a whole lot of details and potentially vital information to a finished photograph. This is quickly evident when a photo is enlarged by cropping out 33% or more, or printing to sizes above 8x10.
Back to the sensors. First, there are TWO types:
CCD (charged-coupled device) which is currently the most common type of dSLR (Digital Single Lens Reflex) sensor, and some of the more expensive (pro level) high megapixel compact cameras (particularly the underwater & sports action models priced over about $600 MSRP).
CMOS (complementary metal-oxide semiconductor). Which is less expensive to produce. Initial implementations took advantage of CMOS' on-chip electronics to make less expensive sensors, better low light sensors, and ultimately a less expensive camera; however, they are a whole lot more noisy (pixelated) when enlarged.
Second, there is often much confusion on this topic because there are so many different size options, and so many trade-offs relating to image noise, diffraction, cost, size, weight, and even power consumption.
In general, professional level cameras are usually 'Full Frame,' which is considered equal to a 35mm film size (ie. 'the standard'). That sensor (and camera body) is, at this time, usually priced well above $2,000 (not counting the lens cost). Just about every other camera under that magic price point is a 'crop sensor.' This is important for the type and size of prints you'll need... and how much cropping (eliminating of the captured information to gain a finished photograph) you plan on doing, and the type of shooting you'll be doing.
There are MANY different 'sizes' of crop sensors, which can make this a very complex issue. In defense of the larger crop sensors the 'pro-sumer' (Nikon & Canon) have, one might initially think that throwing away image information is never ideal, however can have some advantages; especially in wildlife & sports action photography because you effectively gain 'zoom' range with less expensive (shorter) lens. Example: a 100mm full frame lens would be equal to a 150mm on a Nikon dSLR, 160mm on a Canon dSLR, 200mm on a 4:3, 300mm on the next step down, etc. etc. etc.
Nearly all lenses are sharpest at their centers, with quality degradation progressively happening toward to the edges of the frame. This means that, unless you can afford the 'best glass' (professional level, low f-stop, large to massive, expensive lens), and a body over the $2,000 average... you will be stuck with a 'crop sensor' camera that gives you an additional magnification factor. The question then becomes just how big of a sensor can you afford, and what do you need for your photographic goals?
The majority of the dSLR's on the market ARE CROP SENSOR CAMERAS. Hundreds of millions are sold every year, compared to just thousands of full frame cameras. Nikon's generally have a 1.5 crop, Canon's (which is what I shoot) have a 1.6 crop. Olympus, Fuji and Kodak all teamed up to create a 'standard' 4/3 system, which has a 2X crop factor compared to 35 mm film. Camera phones and other compact & micro-compact cameras use sensor sizes in the range of ~1/4" to 2/3".
Knowing your camera's crop factor is important for a variety of reasons. First, so you can compare 'apples to apples' when shooting with someone else. If the other person is using a full frame camera, shooting portraits 12 feet away from the model, with an 85mm lens; and you are using a Canon 1.6 APS (crop sensor body) then you'd be about 10 feet away with a 50mm lens to capture the same 'area' (50x1.6=80, and you gain (or lose) about 2.5 mm every foot closer (or further) from the subject). At low ISO, both cameras shooting in RAW, with proper post-processing, the average person won't be able to tell any difference up to at least a 16x20 print size. The higher the ISO, the worse the lighting, the larger the final print size, the more a person can tell a difference.
There is a marriage between megapixels and sensor size, they work synergistically with each other. The megapixels are effectively the number of dots of RGB (Red Green Blue) pixels being captured; whereas the sensor is the size and amount of room those megapixels are confined to. Many refer to this marriage as 'bucket size' - which is accurate, but I don't think demonstrative enough. Try this: consider having eight people in a semi-truck (full frame) vs moving truck (pro-sumer 15./1.6 crop sensor) vs cargo van (4:3 ratio) vs large car (8.8x6.6 high end compact ) vs economy car (7.18x5.27mm mid-range) vs ultra-compact clown car (5.27x3.96mm). Remember, 8 people are still 'in' the vehicle. You can quickly see that details will be lost the smaller you go, merely because of space.
All of PS (Point & Shoot) cameras have serious crop-sensors, meaning that what they can actually 'see' through the lens will be 'cropped' to what is actually captured. If you will never be printing above 8x10, or cropping the photo drastically after it's taken, and can work in optimum light... then a PS is much better than no camera at all (or an even smaller sensored phone camera).
Medium format and larger sensors exist, however these are far less common and currently prohibitively expensive. The Hasselblad H4D-200MS Digital Camera (Body and Digital back, not even counting any lens) costs a whopping $43,995! These will thus not be addressed here specifically, but the same principles still apply.
In summary, a cropped sensor is forced to use a wider angle lens to produce the same angle of view as a larger sensor; and, this can degrade quality. Smaller sensors also enlarge the center region of the lens more, so its resolution limit is likely to be more apparent with lower quality lenses. Larger sensors 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 (of colors & light); ie, more data is captured & saved (especially in RAW format). Dynamic range is essentially the number of tones which a sensor can capture, to define texture, edges, and sharpness... to make the subject discernible from background. Since larger pixels have a greater volume (ie. the bigger bucket or larger vehicle) they have a higher dynamic range. Further, larger pixels receive a greater flux of light & color photons during a given exposure time (at the same f-stop), so their light signal is much stronger (thus less noise).
Which brings us back to megapixels. The sensor with the higher pixel count will produce a cleaner looking final print. This is because the noise gets enlarged less for the higher pixel count sensor (for a given print size), therefore this noise has a higher frequency and thus appears finer grained. The resolution of a digital sensor is determined by the number of megapixels, not the overall size of the sensor. Squeezing 10 megapixels onto an 7.18x5.27mm sensor chip (a phone camera), does indeed provide 10-MP resolution, but the noise will be much higher than, for example, the Canon 300D (Rebel) a 6-MP DSLR with a 22.7x15.1 mm sensor. Noise in a digital image is manifested as speckles all over. This is particularly enhanced and worse at higher ISO settings, an understandable phenomenon when you consider that higher ISOs are achieved simply by increasing the amplification of the sensor light signals. It is the same as turning up the volume on a radio for a distant station that is already not clearly coming through as good as it should (or could).
I hope this helps... (and isn't too boring).