Regarding the RF 100mm -500mm lens,
according to Gordon at CameraLabs, here is the breakdown of apertures across the zoom range. Not too disappointing:
- f/4.5: 100mm - 151mm
- f/5.0: 152mm - 254mm
- f/5.6: 255mm - 363mm
- f/6.3: 364mm - 472mm
- f/7.1: 473mm - 500mm
It would sure be nice if they had a lockout at 473mm...I'd give up a tiny bit of range for not having to worry about losing 1/3 stop.
I guess it all depends on why you want to limit yourself to f/5.6.
You can just monitor the f stop through the viewfinder as you zoom. When you go above your max preferred f stop you can just back it off a little bit. This approach works for any chosen f stop you don’t want to exceed on any lens, mirrorless or not. This can be done easily by setting the f stop to f/5.6 before you zoom, and back off when it changes to f6.3. You could apply this method to the EF 100-400L II if you didn’t want to go above f/5.0. You are unlikely to notice the 1/3 stop change in most cases as others have noted. This has been my experience with the M series 55-200, 18-150, and 15-45 that all go to 6.3 at the long end.
Changing the exposure level increments to 1/2 stops to prolong the f/5.6 range as has been suggested. It just takes a bit longer to switch since the next increment is 6.7 instead of 6.3. On the EF-M 55-200 this extends the 5.6 range from about 150mm to 190mm. But the EV level metered is still the same so it decreases the shutter speed as you zoom or you’d be underexposing by a little bit in the extended range. In Av mode the shutter speed now changes at 150 to account for the EV change. So, have you gained anything? You really aren’t letting any more light in by prolonging f/5.6, that’s just physics.
The same thing happens on the 100-400L II. At 1/3 stops, the f stops are f/4.5 from 100-135, f/5.0 from 135-300, and f/5.6 from 300-400. Switching to 1/2 stops gives me f4.5 from 100-300 and f5.6 from 300-400. Extending the f/4.5 range may be tempting, but you just get a wider range of shutter speeds and ISO in this mode because it hasn’t changed the amount of light hitting the sensor. It only affects the settings the camera can choose to properly expose it.