Hi, I am a bit confused regarding using FFT to calculate the 1D single slit diffraction pattern.. Let's take the simple setup shown here: http://electron9.phys.utk.edu/optics421/modules/m6/images/sslit.gif" [Broken] If I wanted to do a quick calculation using Matlab, I would create a 1xN matrix of zeros everywhere, and 1 in the middle wherever the slit is located. Then I can simply use the fft function from Matlab to get the diffraction pattern. But what I really want to know is the diffraction pattern as a function of x (vertical x-axis) and determine the actual spot size of the central beam for example. Let's say that I decide that the slit matrix at index 1 corresponds to x=-1 mm, slit matrix at index N corresponds to x=1 mm, and the slit is actually located between -0.1 mm and 0.1 mm (whatever indices these correspond to). Is there a correspondence in the x values for the fft of the slit? Does the value of the fft of the slit at index 1 corresponds to its value at x=-1 mm? Or how does it work? Sorry for the very confusing message.. I hope someone can help. Thanks!