Matt asked this question on another thread, so to keep it OT, I will post a few thoughts here.
With film cameras, when we wanted to make photographs in low light, we used "fast film" or ISO 800 (say) film. The ISO is a standard measurement for sensitivity to light - the higher the number, the more sensitive, so film that needs less light (and thus allows one to use "faster" shutter speeds) for desirable exposures has a higher ISO reading.
Similarly, with digital cameras, the sensors are the equivalent of the film, and they are made to be optimally sensitive to a certain amount of light that is similar to the amount of light required for desirable exposures on traditional ISO 100 or ISO 200 films. Most of these sensors will work with less light, and either with Auto settings or by manually changing the ISO settings of the camera, one can reduce the amount of light required for desirable exposures, and thus speed up the shutter speeds.
Even with ISO 100 film, however, the film emulsion - the chemistry that was activated by exposure to light, is made of very fine particles. The higher the ISO, the larger the particles. If you make large prints from even an ISO 100 film, a close inspection reveals the grain. Even with small prints from an ISO 1600 film, the grain is very evident.
So it is with the digital equivalent. The sensor is a computer chip that converts light energy into electronic signals. Just like in radio systems, for instance, these signals are prone to noise - interference if you like. This interference can come from other energy sources (IR light and heat, other electronic signals in the vicinity - the other components on the miniaturised camera - the memory card, the zoom and focus motors, etcetera to name a few). Thus you have a signal to noise ratio that describes how similar in intensity these two energy sources are. At their designed optimum light sensitivity (taking photos in normal sunlight conditions) the ratio is extremely high. At very high sensitivies, the amount of energy from the light readings is getting to be much lower and thus the noise energy is starting to get very similar to the signal energy, so we start to notice the noise - if it were a radio signal we would hear the buzz. In the digital image we see strange colours and discontinuities in the tones in the image.
Most of us have seen it, I am sure, if we have spent any time looking at our own or other people amateur photography.
Here are some things that affect noise:
1. how dark the image is - noise is worst in shadows, because surprise surprise the energy levels in the dark are lower
2. how small the camera is - the smaller and closer the components, the more prone to one component affecting another, thus increasing the noise (thus digital SLR cameras will generally have much less noisy images in sub-optimal light conditions than compact cameras)
3. when images are extremely overexposed - a really
bright area in the image creates huge energy flows in parts of the sensor affecting the parts of the sensor that had low energy flows
4. how much image compression the camera software or settings make - image compression results in loss of image quality adding noise to the image, as does image editing and adjustments, unless you have professional
tools and skills
5. the amount of effort, development and technology that has gone into the design of the camera
6. chromatic aberation is another noise effect from the optics - the smaller the optics (the lens), and the less sophisticated it's design, the higher the purple or red fringing in areas of high contrast.
So, that is what noise is and why it is caused. But why do you care?
Well, if you take photos - particularly those in low light situations, your photos may not look as lovely (eg. that beautiful sunset) as you had hoped for - you will see bountiful noise in those sunset and weather shots shown with the TV news.
If you are only going to display them on your PC at low resolution, this may not bother you. If you expect to publish them, print them, or sell them, then this could be a show-stopper.
Sites like www.dpreview.com and others review cameras (not all of them because there are 1,000s of them released each year), and noise will be one of those things that they assess. Taking some photos with the camera you are thinking of buying in low light conditions and taking them
home and looking at them in detail on your PC is a way of determining if the levels of noise are acceptable.
Digital cameras allow us much more flexibility in working in low light conditions - we don't have the change films. As a consequence, we have to worry about noise.
Here is a DP Review article on the subject with some images, for those that wish to read more.
Cheers
Andrew.