An optical system consists of 2 thin lenses, separated by a stop. The focal length of the first lens is 20mm and the diameter of the first lens is 15mm. The stop is located 5mm to the right of the first lens and has a diameter of 12.0mm. The second lens, having a focal length of -12mm, is located 8mm to the right of the aperture with a diameter of 5.0mm. Using any form of paraxial ray tracing, determine for an object at infinity and image at the paraxial focus:

1. Based on surface diameters, which surface functions as the aperture stop?
2. What is the size and location of entrance pupil?
3. What is the size and location of exit pupil?
4. What is the system F/#?