How to Estimate the Noise of an Image / Estimation of the Noise in an Image? I want to know how to estimate the noise of an image?
Also is this represent the camera's noise?
Does anyone know how to estimate the noise of an image or camera?
Update:
Especially, I want to know about the A, B, C from as following.
How Can I plot these A, B,C curve?
Also I want to know what is C. What does C mean?
There are various methods to estimate the noise of a signal (or adapt a threshold to be used later in PCA for example). 

There are adaptive methods and statistical methods. Statistical methods employ a statistical model of the noise and estimate from the data, while adaptive methods iteratively filter the data until a certain threshold of (reduced signal) accuracy is reached. These are just generalities of the most used approaches. 

The following references deal (mostly) with images from cameras:


Noise Estimation from a Single Image
Automatic Estimation and Removal of Noise from a Single Image
A noise-estimation algorithm for highly non-stationary environments