3) Variation between lenses and an important lesson learnt

Lenses are assembled on a production line, albeit with highly skilled people. The production line process means that no two lenses are quite the same; some variation will always exist. In terms of Autofocus adjustments, it is interesting to note that Canons’ published tolerances are ±5 points, while those of Nikon is ± 10 points! Other manufacturers such as Sigma and Tamron are ± 15 points. So it would seem that the tighter the tolerance the more expensive the lens. As a word of advice, when you purchase a new lens, check the AF and if it is out by more that this amount, I’d return it and ask for another copy. I took delivery of one of the first 500mm PF lenses in the UK but was horrified how poor the focus was. I got a series of soft images that were very frustrating. I returned the lens to Nikon UK who were amazed that it had left the production line in that state. Nikon UK confirmed the lens was out by more than 10 points and they rectified this. Fortunately, such lens adjustments can be made by the manufacturers via the electronic contacts at the base of the lens. So I suggest you test your new lens carefully when you receive it and if you are dissatisfied with its performance, return it.