 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 cm from the lens 1 upon V minus 1 upon U by the formula. How far is the object placed from the lens? So, 1 upon U equal to negative 1 by 10, positive 1 by 50, 30 is the LCM 2 minus 3 is negative 1. So, U is negative 30 cm, so the object is placed at a distance of 30 cm from the lens.