If ISO is not a factor at f/11 in gloomy weather like today’s, then it probably won’t ever be. The only challenge with the 600mm f/11 so far is keeping away from MFD, to which end I’ve set the shutter to focus priority.
Even in low-contrast situations, AF doesn’t balk, nor does it hunt in the wrong direction (hear that, Nikon, now #3 or 4 and still dropping?). The lens wasn’t bad at all on the RP, but is noticeably better on the R.
The next obstacle is to avoid being tempted by the 800mm while I’m waiting for the next shipment of 100-500mm. A hood is on the way, and the extender will get another try when it arrives. But my thinking is that the lens doesn’t really have the acuity to benefit from magnification.
No problem, like many of Sigma’s current lenses, it’s great for what it is, and real bang for the buck.