Problem with gaussian MF

Hello, I want fuzzification an image with gaussian membership function. But i don't know what is the best or optimal or basic C and sigma? Thanks

clc;
clear ;
close all;
I = imread('gc13.jpg');
    img = I;
C = mean2(img);
SIGMA = std2(img);
           X =  exp(-(img - C).^2/(2*SIGMA^2));
imshow(A);