 A concave lens, concave lens sign conventions are negative focal length, object distance is anyway negative, red dot image is always negative, clear. A concave lens of focal length 15 forms an image 10 centimeter from the lens 1 upon v minus 1 upon u by the formula, how far is the object place from the lens? So 1 upon u equal to negative 1 by 10, positive 1 by 15, 30 is the LCM 2 minus 3 is negative 1, so u is negative 30 centimeters, so the object is placed at a distance of 30 centimeter from optical center of the lens.