Images formed by a thin lens or a spherical mirror are characterized by the equation:

         1/do + 1/di = 1/f      

using the following sign conventions:

• do = object distance is positive for an object on the side from which the light is coming; negative if the object is on the other side.
• f = focal length is positive for converging lenses and concave mirrors; negative for diverging lenses and convex mirrors.
• di = image distance is positive for real images and negative for a virtual image:
• For a lens: di is positive if the image is on the opposite side of the lens from where the light is coming; negative if on the same side.
• For a mirror: di is positive if  the image is in front of the reflecting surface; negative if the image is behind the mirror..
  

Note: For an object at infinity (a very large distance away), the term 1/do is effectively zero, so in this case the above equation reduces to
           di = f
In the symmetrical situation where the rays are parallel when they leave an optic system,  the image distance is infinite, the term 1/di is effectively zero,  so the equation reduces to
            do = f