This is just a very quick mention about an aspect of fine tuning the focus of your lens to your camera body which many of you may already know. Mongo will mention it just quickly for those who may not.
When you fine tune the focus of any lens on camera bodies that have this facility, it is NOT enough to do it to the bare lens alone if you also intend to use it with converters.
Simple example, the Nikon 300mm f4 AFS Mongo tested and posted recently in this forum had NOT had an auto focus fine tune carried out on it at that time. Mongo reported excellent sharpness for the bare lens and also with the 14EII converter BUT said the lens did NOT work well with the 20EIII converter. That last finding proved to be very INCORRECT. The lens does in fact work extremely well with the 20EIII.
It was not enough to tune the bare lens.
Since then, Mongo has fine tuned the auto focus. Mongo did the auto focus tuning for the bare lens and each combination of the lens with different converters. Logically, adding any other lens glass to a bare lens (like a converter) changes the characteristics of that lens/converter combination. The camera sees the lens and converter combined as a new or different lens - and indeed, it may well have different optical characteristics as a result.
To illustrate the differences, the adjustments needed were as follows:-
+4 for the bare lens,
+9 with the 14EII converter,
-15 with the 20EIII
With such a large adjustment difference to achieve spot on focus, it is no wonder that the original (uncalibrated lens) results yielded very poor outcomes for the 20EIII which in real terms, required over 20 points adjustment (at least 15 of which were in the opposite direction). The results are now very sharp for all three scenarios mentioned above.
So, for those who have not fine tuned the focus of their lenses (and have that ability on your camera body), .......what are you waiting for ???
For those of you have just fine tuned the bare lens but may use converters, you need to fine tune every combination separately if you want results. Your camera does not mind. It happily stores each combination separately and call it up an applies those adjustments every time you pop the lens on and have the AF turned on.