What is the value of the standard deviation $\sigma$? I am using the following formula to add the additive Gaussian noise.
noise= (x/255)*(randn(size(image))), where x is the variable and randn is random value generator. Intensity range is $0-255$. But, what is the value of the standard deviation $\sigma$ here? Is it x or (x/255)?
randn gives you Gaussian noise with std. dev. of 1. 

(x/255)*(randn(size(image))) gives you a noisy image of std. dev. (x/255)