Please help.

1) In measuring focal length Measuring the Focal Length of a Positive Lens Using Displacement Method, If the distance between the object and the screen is at least four times of the focal length and when we move the lens, we can get a clear image twice at different points as shown in Figure 2-3. SI SI P' $2 $2' D Figure 2-3 Schematic of thin lens imaging Correspondingly, we get the two following equations: 1 . 1 (2-1) f S, S , (2-2) Using the conditions: D= S, +s, =S, +$2, $2 =s, +d,s, =s, +d, we can get the formula: D' - d2 (2-3) 4D Explain why should the distance between object and screen be at least four times of the focal length