Textures can create the illusion that certain small deformities exist. This is often used for scratches and things that are not always modeled. https://www.marmoset.co/wp-content/uploads/2014/01/lenstextu...

Scratches like these could sometimes be painted directly onto the texture - or maybe sculpted onto a higher-resolution model and then "baked" down to the lower-resolution model. In this step lots of data from how light bounces off the high-resolution model can be imprinted to a texture and re-used in a lower-resolution model.

If you want to learn more the terms to be googling are "normal mapping" and "bump mapping"

In this article - If i was to guess; all the pictured spheres are perfectly spherical, and just textured differently. https://www.marmoset.co/posts/physically-based-rendering-and...

Also - heres an interactive model of the lens: https://www.artstation.com/artwork/kl0A6 The "Layers View" button in top right gives you a neat view of the model with/without certain texture layers