Just to contribute my own experience with lenses up to 600mm on my D800 and A7RII:
- The traditional shutter speed rules of thumb are general guidelines - with my 400/5.6 ED I can get away with 1/800 sec (1/2xFL) with good success and with my A7RII using IBIS 1/FL (1/400) will generally give me very very good results (sharp at 100% viewing of the image)
- Hand-held ability varies from day to day - sometimes I have more high-frequency shake than other times and thus results vary.
- As others have said, atmospheric conditions [turbulence level (fluctuation level), the physical scale of the turbulence (eddy size), and turbulence type (wind shear vs. thermal vs. both), etc.] can play a key role at any significant distance. There is not "safe" time of day for reduced turbulence, it can happen at any time depending on what type of turbulence it is.
- Lens thermal stability. As Bjorn pointed out, with long lenses, just as with telescopes (reflector and refractors) if the optic is either out of thermal equilibrium or is very hot/cold relative to the surroundings this will have bad effects on the image quality
- Some lenses are myopic - i.e., even with perfect atmospherics, they tend to be sharper at closer distances than at "infinity"
- Focusing accuracy: All of my telephotos give poor image quality with very very very minor changes in focusing. My D800 is nearly useless with my 400 and 600mm lenses as the OVF is totally inadequate and the poor LiveView implementation is not accurate enough. Only my A7RII is really good enough with LiveView to get optimum focus. The D810 is reportedly much better with LiveView.
- My long lenses have good days and bad days depending on the absolute temperature. I.e., even if the lens has come to thermal equilibrium sometimes they don't like being very cold or very hot
My 2 cents.
- John
