matlab - Var of added gaussian noise -

I basically have this:

  I = im2double (imread ('lena .png ')); Add Gaussian Voice J = Emois (I, 'Gaussian', 0,0.05);   
 Now both of the undegraded images have  i  and noised image  j  how can I guess that variation used for Gaussian noise < Code> 0.05    
 
 
 You can get noise wave through subtraction. You can get sample variation with  var () .  
 Of course, sample variation is only an estimate for actual variance.