power 4dTwo lenses have power of (i) + 2 d (ii) -4D. What is the nature and focal length of each lens? b. An object is kept at a distance of 100 cm from lens of power -4D. Calculate the image distance.Transcript. Question 39 If the power of a lens is − 4.0 D, then it means that the lens is a (A) concave lens of focal length −50 m (B) convex lens of focal length +50 cm