SURF: How could we get the value of sigma from the keypoint radius
This value s = size*1.2f/9.0f is not montioned in the bay's article scale= L*0.4 or 
scale= L* 1.2/3 any one can explain me this part??
In the SURF technique, and more precisely within the feature description stage, the authors have stated (if I understand correctly) that the description will be performed in a area of 20 times sigma. Sigma represents the scale on which the keypoint was detected.
Sigma = 0.4 x L where L = 2^Octave  x level+1. If we use the OpenCV implementation, the DetectAndCompute function computes, with the value of Keypoint.size, the radius of the circle surrounding the keypoint. 
My question is : How could we get the value of sigma from the radius value  ?