A candle is placed 1.8 m from a screen. whereshould a lens of focus length 21 cm (0.21 m) be placed between the screen and the candle to prduce a sharp image screen? The only formula i could think of was 1/f=1/do+1/di I've now spent 3 hours on this (test tomorrow) and still don't know where to start. I have that first formula and the only other formula we've used had to do with magnification and object height. I am truly lost.