The scale space for SIFT algorithm is constructed by convolving an image with a sequence of gaussians as below:
where G(x,y,s) is the gaussian kernel (s is the scale parameter) and I(x,y) is the image.
My question is: what is the first value of scale s that we use to create the first laplacian image in the first octave?
My understanding is that scale = 0 represents the original image itself. How do we select the first scale value?