I thought I'd make it easy for myself and went to Levens Hall deer park to try the Olympus 100-400mm on something a bit bigger than garden birds.
The 800mm equivalent focal length allowed me to get within range without spooking the deer.
This image taken at 349mm (nearly 700mm FF equivalent) is tack sharp even at 1/500th second shutter speed with the help of the in-lens stabiliser.
|Olympus EM1 MKII with Olympus 100-400mm. 1/500th sec, F6.3, ISO 250, @349mm|