You should be vectorizing your code:
img = (-0.18 ./ (-0.28 ./ (45.39 ./ double(img1) - 1))+1) * 5.3;
The result will be something that is infinity where img1 is 0. How do you want to handle that case?
Your expression works out to be of the form A/x + B for constant A and B. That inherently trends towards infinity as img1 becomes small.
