Please help as I do not know how to read the exact data on the measuring scales.

During the experiment, we use auto-collimation method to measure the Focal Length of a Positive Thin Lens. As shown in Figure 1 below. An object P is placed on one side of the convex lens. When it is in the focal plane, any ray from the object as refracted by the lens would change into parallel ray. After reflected by the plane mirror and again refracted by the lens, it is converged in focal plane of the lens. The distance between lens and object is the focal length of the lens: f = s Experiment setup LEPO-54-2 LEPO-54-3 LEPO-54-4 LEPO-54-4 Figure 1-1 Photo of experin Figure 1-2 Configuration of components 1: Bromine Tungsten Lamp S (LLC-4) S: Flat Mirror M 2: Object Screen P (LEPO-15) 6: 3-D Adjustable Post Holder (LEPO-17) 3: Convex Lens L (f=190 mm) 7: Optical Rail (LEPD-54) Figure 1 Schematic of thin lens auto-collimation 4: Two-axis Tilt Holder (LEPO-8) During the experiment, the lens is moved back and forth until the image is clearest and with the same size as the object. The location of L is shown below: 6 7 8 9 601 2 4 6 8 9 701 2 3 The location of P is shown below: 2 3 4 5 6 7 8 9 80 3 4 7 8 9 901 2 Reverse L to exchange its front and back surfaces, repeat the same steps as described above. The location of L is shown below: 5 6 8 9 60 1 45 8 9 701 2 3 4 5 6 The location of P is as shown below: 701 2 3 4 5 6 7 8 9 80 3 4 17 8 9 9 Calculate focal length: fi = $2 - S, f2 = $4 - $3 f = litfi 2 Please also explain we need to redo the test by reverse L to exchange its front and back surfaces and Explain the relationship of size and orientation of image P' and object P in Fig.1