In the case of multiple thin lenses, the overall focal length is related to the lenses’ individual focal lengths:
Therefore, if two converging lenses next to each other have the same focal length, the combined would be half the original. The same formula applies to diverging lenses, but with different signs for the image.
The quantity of the lens is known as its power. The unit of 1/meter is known as Diopter(units of this ‘power’), as the focal length is measured in meters. For diverging lenses, the power is a negative value.
Essentially, power for multiple lenses is simply added together.