1. The problem statement, all variables and given/known data An object at a distance a1 in front of a convex lens with the focal length f1 is imaged at a distance b1. A second lens at the distance d from the first lens with the focal length f2 is then imaging this Image with the image distance b2. a) Give the image distance b1 as a function of f1 and a1, as well as the image distance b2 as function of d, a1, f1 and f2. b) What is the focal length of a System of two lenses which distance d approaches zero? c) Which focal length must the second lens in b) have to compensate the action of the first lens? How is such a lens called? 2. Relevant equations Lens equations, fs=(f1*f2)/(f1+f2). 3. The attempt at a solution For b) I have found the equation given above. How can I figure out c)? I think it is an afocal system, is that correct? For that, the focal length must be f2=d-f1?