f/5.6 at 1/1000 and ISO 100 requires an EV 15 subject illumination. That's basically full sun. Heavy overcast or sunset is EV 12, so need ISO 800.
You should have decent DoF at 500mm on APS-C or FF from, say, 30m out. At closer range you'd likely need to bump up the f stop which will also bump up the ISO. At 10m, you'd have about a +/- 5cm DoF, at f/11 it's +/- 90 cm. f/11 at 1/1000th would be EV 17 (4x more light needed), so you would need ISO 400 in daylight or 3200 at sunset / heavy overcast / shade.
Personally, I think f/5.6 is pretty usable if you're willing to shoot ISO 200 - 800 regularly. I make do OK with the Tamron 150-600 f/4.5 - 6.3. But if you are used to shooting 500 f/4 or 400 f/2.8, you will need to adapt. 1/1000th used above is actually not that fast for birds, I sometimes want to use 1/1250 or 1/1600, which will indeed push ISO pretty hight. Fortunately, I shoot the d850 which handles it OK.
The main drawbacks will be the autofocus. f/4 is a magic number. Slower than that and you're down from 99 cross sensors to 45 cross sensors (on d850).
I do not think using the 1.4x TC will work out that great. I've seen some reviews that show it doing well, but f/8 is going to be a drag. A tc-14e on an f/5.6 lens drops the d850 to 15 focus points (9 selectable) with 5 cross sensors.
You will also lose 3d tracking when the TC/lens combo is over f/5.6.
That all said, I'd love to shoot one!
Marc
I agree that you will at least loose
some AF performance with a f5.6 lens compared to a f4 or f2.8.
And it will be a bit more significant in low light I guess. The focus motors in the 500 PF seem pretty snappy though.
When it comes to shooting action, a larger f-stop is never a bad thing. As an example I was shooting with the D500 + 300/4 PF the other day.
It was
2 hours before sunset, sunny with no clouds. The light was strong with distinct shadows but not too harsh like in the middle of the day at EV15.
The exposure settings I got were 1/1000s, f4 at ISO800 (EV11@ISO100). Even though the subject was not the fastest (a big owl) the 1/1000s was on the long side for freezing all action of the hunting owl.
One hour before sunset, with the sun behind some thin clouds, settings were 1/800s, f4 at ISO3200.
The last 30 to 15 minutes before sunset, with a bit less clouds again, settings were 1/500-1/800s, f4 at ISO3600-5000.
For the "sitting bird" shots I could use a slower shutter speed and lower ISO of course.
But the bottom line is that with an f5.6 lens on the D500 I would have been struggling even more with the short shutter speeds and ISO.
So for my needs, the 500mm/5.6 PF would be a better fit on a FF camera. Your milage might vary of course.
