Optics - Focal length question

[Lux0r]

Homework Statement

For a lens with a focal length f, find the smallest distance possible between the object and its real image.

Homework Equations

1/s + 1/s' = 1/f

The Attempt at a Solution

I tried plugging in different numbers for the solution, but I'm not sure how to reach the solution..

The answer is s + s' = 4f

 

[ehild]

What is the distance D between the object and image in terms of s and s'? Find the expression D(s). Differentiate D with respect to s and solve the equation D'(s)=0.