If your image has 640 x 480 pixels, and the center is at the pixel 320 / 240:
img = rand(480, 640);
y = 1:size(img, 1);
x = 1:size(img, 2);
c = [240, 320];
D = sqrt((y.' - c(1)) .^ 2 + (x - c(2)) .^ 2); % >= R2016b
With older Matlab versions:
D = sqrt(bsxfun(@plus, (y.' - c(1)) .^ 2, (x - c(2)) .^ 2));
This is the distance measured in pixels.
