Hi Naif,

The thickness of surface 2 is not the focal length of the lens. It is the back focal length, which is the distance from the vertex of the last exit surface of the lens to the image plane. This is close to -- but not equal to -- the focal length because for a thick lens the focal length must be measured from a theoretical surface called the principal plane. (I suggest you Google that.)

The real lens you have inserted is in fact a real lens offered by a vendor. In this case, it is P/N 47-182 from Edmund Optics. It has a focal length of 254 mm at a wavelength of 10.6 um. This is a lens for IR applications. That is the usefuness of the lens catalog -- you can actually buy the lens. (One-off custom lenses are VERY expensive.)

See https://www.edmundoptics.com/p/254mm-dia-x-2540mm-fl-uncoated-znse-plano-convex-lens/7209/

Kind regards,

David

Hi David,

Thank you very much for this crystal clear explanation.


I appreciate your amazing contribution to this massive platform.

Best Wishes


Naif