According to the formula of a small lens, which of the following is correct is about the focal length of a lens?

• According to the Cartesian sign convention, all distances measured from the lens's optical centre to the right side are considered positive and to the left side are considered negative.
• Since the focal point of a diverging or concave lens lies on the left side, its focal length is negative.
• Since the focal point of a converging or convex lens lies on the right side, its focal length is positive.

Therefore, only the last statement is correct.