Very new to Digital SLR Cameras. What is noise in referance to Digital? How does

Question:

Very new to Digital SLR Cameras. What is noise in referance to Digital? How does it manafests itself?

How do you avoid it?
How do you clean it out of your shot/photo?

Best Answer - Chosen by Asker: The sensitivity of a camera is typically expressed in either the number of photons or in a measure of photon flux which can be related to human observations, called the Lux. A Lux is a measure of illumination which has a value of 1 lumen per square meter. The Lumen is a photometric equivalent of a watt which is weighted to match the eye response of the "standard observer".

The sensitivity of the human eye varies at different wavelengths and this has an implication of the number of photons equivalent to a given photometric quantity. The conversion to photons in the table above assumes the light is monochromatic yellowish green light with a wavelength of 555nm which is at the peak of the sensitivity of the human eye. For a given minimum sensitivity in lumens the number of photons varies, for example, see below a table showing the minimum light levels discernable by a typical human observer in the various measures.

If a given light signal induces a signal on the camera below the readout noise of the camera it cannot be detected so the total noise of the camera is a useful way to define the sensitivity of the camera.

There are three main sources and they are:
Sensor readout noise
Thermal noise
The noise from the signal itself: photon noise

The total camera noise is the sum, in quadrature, (i.e. the square root is taken of the sum of the various square of the noises)

The website below should give you an excellent explanation, including charts and graphs.