Try to form an image of an incandescent bulb on your 'screen' by changing the distance between the lens and the bulb. Start with the lens about 18" away from the filament. Once you have a focused image on your screen, move the lens closer to the bulb and move the screen to find the image once again. What is the behavior of the image distance? What happens if you position the lens so that the bulb is near the focal point? Bonus: Can you confirm and quantify your observations using the thin lens equation?