In response to NissanPatrol, the lens is the Sigma 100-400 f5-6.3 DG OS HSM Contemporary. I have had the lens for about 6 weeks (purchase second hand only a month old) using it for garden birds during lockdown. I am very happy with the IQ finding it better at 400mm than my nikon 300mm pf at f8. The focus aquisition is certainly quick enough for the subjects I have taken to date and there is no hunting. Just waiting for lockdown to end in order to try BIF. If I am happy at its BIF performance I will sell the 300mm as the sigma is more versatile, has better vr, and is producing better images.