On a computer simulation (Simion - ion simulating program) I've got two cylinders (with a small gap between them) through which an electron beam goes through. I understand that the "lens" refers to the equipotential lines, since they form a bulge at each end, just like an optical lens. What does 'lens diameter' mean? The distance of the gap, or the distance from one end of the 'bulge' to the other end? That's the basic question I have - the rest is more details on the rest of my task if you want to help. I need to re-construct a graph for work, it shows 'lens diameters' on the y axis, going from +2 to +10 above the x-axis, then again from +2 to +10 below the x-axis (what does that mean anyway, why both above and below?). Then it shows "V2 / V1" on the x-axis.. and the line above the x-axis looks like it's exponentially decreasing while the line below the x-axis looks like it's exponentially increasing.. the lines are labeled F1 and F2, for focal lengths. The top and bottom of the x-axis are mirror images. So.. I can set which voltages I want, and I can figure out the focal length of the beam through the equation 1/P + 1/Q = 1/f. What does it mean by 'lens diameter?' How do I measure that? And how can this be an x vs. y graph if there are three terms? The diameters, V2/V1 ratio, and the focal length lines. I'm confused on how you can have three of them on a two-part (x and y) graph. Any help or guidance in the right direction would be greatly appreciated! Note: I've only taken general physics I and II, so my knowledge isn't very extensive, just kind of got thrown into this.