A thin lens is a simplified theoretical system that has no curvature or refractive index and zero thickness. Of course, a « real » lens is made of a material that has a certain index and curvatures. In this case, one can apply the paraxial formulas specific to thick lenses to calculate the object and image focal lengths (these formulas are reported here: https://www.gatinel.com/paraxial-power-of-the-iol/ within the framework of the study of the paraxial properties of the intraocular implants). Once these distances are known, the thick lens can be replaced by a thin lens whose vergence is calculated using the position of the foci (the inverse of this distance if the lens is in the air).