Hi, I have a very simple test bench set up with a HeNe laser, followed by a spatial filter, then an Iris and then a couple of lenses. I can observe many fresnel rings on my image plane. Changing the size of the iris, and its distance changes the number of of rings. I would like to know why this happens. Is there any way I can get uniform illumination from my laser? Is it because the spatial filter pinhole diameter is too big? It has a 40X, 0.25NA objective.