This fixation with mount diameter is nothing more than a distraction. There are so many other factors involved in lens design, and so much room for improvement – none of which have anything at all to do with the mount diameter. The following are just a few off the top of my head:
Breathing – the change in FOV when moving the focus point, especially disconcerting to filmmakers when pulling focus.
Focus shift – the change in focus that occurs when focusing wide open, then stopping down.
Fly-by-wire focus that is not linear and has no hard stops (again, esp. undesirable to filmmakers). Olympus has come up with a clutch manual focus solution. In the menu of the X-H1, it is possible to enable linear focusing and even change the direction.
Onion ring bokeh (5 years ago, Panasonic announced they’d reduced the peaks and valleys of aspheric elements that cause onion rings to 20 nanometers; recently, Sony claims to have halved that number) When it comes to my own work, I doubt if more than 15% of the image area in any of my clips is ever actually in focus, since I seldom shoot with apertures smaller than f/4. So the way a lens renders those out-of-focus areas is of vital importance to me – of equal importance to the in-focus areas.
Weather-sealing, made for extreme weather conditions.
I have concerns about fluorine coatings, about which Roger Cicala says, “we despise the new fluorine front and rear coatings and consider them a significant step backward in durability.”
Silent AF, once more, of great importance to videographers.
We already mentioned bokeh – the distinctive way in each lens renders out-of-focus highlights and which, for cinematographers who truly care about the texture of those blurry bits, which, as I said earlier, may occupy as much as 70% or more of any given shot – is the unique ‘fingerprint’ of a lens. But other characteristics, such as flare, glare and coloration may be of equal or even greater importance to filmmakers. And unlike pin-sharp corners – which a great many laymen seem to think is the indisputable advantage of larger mount diameters – the qualities I mentioned are resolution independent: meaning that the viewer will notice and respond emotionally to them whether watching on an iPhone, a tablet or a 65” OLED display.
And finally, there continues to be inconsistency in how quickly different lenses by the same manufacturer autofocus, sometimes considerable, sometimes not. Apparently, some Fuji lenses that were notoriously slow now work better with the X-T3. Other manufacturers should continue to publish firmware updates, not only for bodies, but for lenses as well.