1. The problem statement, all variables and given/known data Needan optical sensor to detect a dot sequence on widgets passing 50cm away from the sensor. Want each dot to exactly cover the sensor. If the dots are 10mm in diameter and the sensor is 40mm in diameter, what is the position and focal length of the lens to do the focusing. 2. Relevant equations m=-di/do 1/do+1/di=1/f 3. The attempt at a solution m=-di/50cm 1/50cm+1/di=1/f not sure what number is di, f, or m; if I knew one of these I could finish the problem.