MATLAB: How to output matrix grayscale

Question

I am trying to output my matrix grayscale with imshow(), but its looks like gradient.

My code is:

 s=4;
 A=256;
 N=200;
 i=1:N;
 r=0.1*(i-N/2);
 f=exp(-r.^2/2/s^2);
 M=A*(f'*f);
 imshow(M,[]);

Please, help me to find a solution to this problem

Answer 1

This one?

imagesc(M);