How to Estimate the Noise of an Image / Estimation of the Noise in an Image?
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