What is Camera Noise?
Noise is information recorded by the camera that does not belong to the scene. There are two sources of it:
When increasing ISO, the camera delivers more electrical power to amplify the light and more pixels fails to record properly. At some point, the noise could be more important than the image itself. There are multiples solutions to get rid of it. However, the result is always loosing pixels which translated into a loss of sharpness.
Most modern cameras have an internal algorithm that reduces it when creating the .jpg image from the RAW file coming from the sensor.
If you shoot in RAW, you will have to rely on post-processing software to deal with noise. The best solution is removing it where it is a problem in the picture. Noises will be very visible in darker areas. It is usually less present in brighter part of the image. it is also less visible in more detailed areas.
When the exposition is longer, it occurs by electric current converting analog information to digital. It increases as the heat build-up in the sensor.
Most modern camera removes it by taking an extra picture while keeping the shutter close for an equal time. Noise in the second picture helps to remove it in the first picture.
How to deal with noise efficiently?
Do you need to fix it? As a photographer we are looking to our pictures and we are seeing the flaws. If the picture is interesting, the subject strong, a little noise will not adversely impact it.
How about a black and white conversion? The most distracting thing about it is color noise. Luminance is not that bothering. Converting to black and white means that color noise becomes luminance noise. You will be pleasantly surprise!
If all this fails, local noise reduction is the way to go! I use Lightroom 4 to do so. With the brush, I paint over the section that I want to fix and I use noise reduction only on these sections. Large areas of sky are the place where noise reductions work the best. Loosing details in these areas is just more pleasing to the eye.
Using Lightroom, I applied noise reduction to the right part of the black square in the bottom right of the noisy image. While this is not perfect as compared to the 100 ISO image, This 12,800 ISO does better with noise reduction applied.