This study demonstrates that the actual thickness of a convex lens is a problem that cannot be ignored, then gradually deduces the object image relationship and focal length of a biconvex symmetric thick lens in the paraxial case in air. The specific differences in focal length and image distance between thick and thin convex lens theories are revealed through examples and drawings, and some of the rules are analyzed. The approximate expression of the focal length of a thin convex lens and the approximate expression of the focal length error of an ideal thin convex lens are derived using approximation, and the latter is almost independent of the focal length. This study also derives that the approximate expressions of image distances of a thin convex lens and ideal thin convex lens on the condition of “the ratio of object distance to focal length” is greater than 2, and the error between them has nothing to do with the focal length, but only with the thickness, refractive index and “the ratio of object distance to focal length” of the lens. At the end of the study, the reason for setting “the ratio of object distance to focal length” greater than 2 is that it is difficult to take approximation when the object distance is within 2 times the focal length, and the reason for dividing the convex lens into the thick convex lens, ideal thin convex lens, and thin convex lens lies in strict distinction.