1. The problem statement, all variables and given/known data An optical system consists of a thin lens in air with f = 100 mm. An aperture stop with a diameter of 20 mm is placed 25 mm to the right of the lens. Calculate the numerical aperture of the system. (Hint: Find the size of the entrance pupil.) What would be the numerical aperture if the aperture stop were placed 25 mm to the left of the lens? 2. Relevant equations NA = h_e/f h_e is the height of the entrance pupil 3. The attempt at a solution I've drawn a picture, but I can't figure out how to find h_e. (The hint was rather useless given h_e appears in the relevant equation!) I know the entrance pupil is the image of the aperture stop in the object space (so you can actually see it?). Help is appreciated.