1. The problem statement, all variables and given/known data A convex lens has a radius of 20mm and a focal length of 100mm. A point source of light is placed 500mm behind the lens and a screen is placed 120mm in front of it. Calculate (to first order) the size of the illuminated spot on the screen. Now calculate the distance at which the screen should be placed to get the best possible image, and estimate the spot size in this case. 2. Relevant equations 3. The attempt at a solution I can picture what is physically going on in this situation, and I think that the reason as to why this is occurring is aberration due to the lens. I'm not sure how I am meant to estimate the size of the spot though -- in what terms etc. Would love any pointers.