Posted this in the Fuji X-H1 thread, but thought it might be interesting to a few here - I've added a few extra bits for clarity in case some reading this aren't Fuji experts (which I know I'm not). We keep hearing that everything about M4/3 is inferior to APS-C which in turn is inferior to full frame etc., but I have to say, this really surprised me.
"Used my Fujifilm X-H1 with a combination of 100-400 and 40-150 at the Zoo with friends on Sunday, and have to say I was left feeling a little disappointing. Love the handling of this camera, but it, combined with the 100-400 in particular gave me no end of slightly soft shots.
It was overcast and slightly drizzly, but even so, the 100-400 hunted back and forth quite badly and in a continuous low burst of say 5 shots, only one would be tack sharp with the rest either slightly soft or really just out of focus. I've used long lenses for many years so I know my techniques not too bad, and despite the conditions and high ISO, my shutter speeds never dropped below 1/250" (with lens and body I/S set to on). Worst of all was when shooting animals though a fence, where the camera would continuously focus on the fence not the subject beyond, despite me manually focusing on the animal first (god it takes a lot of turning the focus ring on the 100-400 to rack the focus back out manually). I tried pretty much all of the AF-C custom settings (1-5) and nothing seemed to be much better than the last. Maybe operator error ? The 50-140 F2.8 was slightly better but even that sometimes missed. Don't get me wrong, when it did nail focus, the SOOC Jpeg were fabulous, even at 6400 ISO.
I then swapped the camera out for my new Olympus EM1X with the 300mm F4 (and occasionally the 40-150 F2.8), and every (and i mean every) shot was tack sharp with instantaneous focus - just bang !. Obviously at the higher ISO's (e.g. 6400) the X-H1 bettered the Olympus for noise control - just, but even so the Olympus jpegs were eminently usable and not that much worse (maybe half a stop). I've seen this behaviour before when shooting birds. In anything other than perfect light I find that long lens photography is quite frustrating on the X-H1 compared to the little Olympus's, (EM1X and EM1 MK II), with the AF lock on (in any conditions) on the Olympus pro lenses being pretty much instant. The biggest thing I noticed was even when the X-H1 acquired perfect focus, the time it took for the lens (100-400) to focus from something relatively near to say 20 m away, took a good couple of seconds - the lens just doesn't seem that quick, whereas no matter where you focus using the 300mm F4 or the 40-150 F2.8, the AF is lightning quick - maybe the Olympus system has spoilt me.
I wish now I'd taken the X-T3 to give that a work out instead of the X-H1, as in my (albeit limited) experience, the AF is a fair bit quicker and more accurate. For portraits and landscapes, the X-H1 is a superb beast, but I don't think I'll be using it anymore for anything wildlife related, think I'll stick to the Olympus. Guess there's a limit to what Fuji can do via firmware to improve essentially the X-T2 AF system from over 2 years ago."