[toni-a]

I often talked about bad lenses and sample variation and need t frequently adjust lenses, since a friend of mine :Tanios  is in the business of lenses maintenance (he used to work for Canon Lebanon but now he has his own business and Canon themselves are outsourcing repairs to his atelier)  Tanios has  explained thoroughly these issues to me and I tried to share what I learnt here, however Roger Cicala does it way better than me so I shared
this good read 

https://www.dpreview.com/opinion/6856813208/roger-cicala-the-difference-between-sample-variation-and-bad-copies-part-1
https://www.dpreview.com/opinion/6856813208/roger-cicala-the-difference-between-sample-variation-and-bad-copies-part-1