ABSTRACT
For example, for a lens with an effective focal length (EFL) of 4.88 mm, if the first object distance is S1,1 = 5 m, we can calculate the corresponding imaging distance as S2,1 = 4.855 mm; while if the object distance is S1,2 = 0.1 m, the imaging distance will be S2,2 = 5.130 mm. Thus, we find a difference of S2,2 – S2,1 = 0.245 mm
in the imaging distance for those two cases. It means that if we have already set the lens and imaging distance for one object (say at 5 m), then to take a picture of another object, which is at 0.1 m, we should shift the lens by 0.245 mm. Therefore, depending on the object distance, we can use the VCM to adjust the focal plane of the lens assembly, in order to optimize the image on the photosensitive device (e.g., complementary metal-oxide-semiconductor [CMOS] or charge-coupled device [CCD]; see Chapter 2), which is usually in a fixed position. The resulting imaging, therefore, is often called the “focus module” and the VCM is called the “focus motor.”
