For years, I have always recommended medium telephoto lenses such as the 100-400mm f4.4-5.6 or f5-6.3. And anyone who is an experienced or pro wildlife photographer will tell you, the sweet spot for photographing small birds is f8, and larger wildlife is f11. For years, I have been a snob and used only the top end gear, such as 300mm f2.8, 400mm f2.8, 500mm f4, 600mm f4, 800mm f5.6, 180-400mm f4, 120-300 f2.8, and these lenses are made by all the top makers and cost thousands of dollars, but I get pro prices.
Due to COVID 19, I have not led an international group photography workshop for 2 years 10 months, but next month, I will be leading my annual Autumn leaves Japan photo tour. And a couple of weeks ago, I recommended to a client to purchase the Sigma 100-400mm f/5-6.3 DG OS HSM, for Nikon. And anyone who knows me knows my personal choice of camera for field work is Nikon, and FujiFilm for medium format. But I will use any camera maker as they are all very similar. I like Nikon because of the twin command dials. But truth be told, I am not happy with the direction Nikon has been going the past few years. In short, my D850, Z7ll and my Z9 are all made in Thailand, “WoW!” But Sigma on the other hand is all made in Japan “WoW!” And I have been mostly purchasing Sigma lenses for the past 8 years, except for a couple of Nikon lenses. Sigma lenses are superior in many ways, firstly they are manufactured locally and the quality is outstanding, and their customer support is splendid. In Japan they even have English speaking customer support, hats off to Sigma. And their turnaround time to repair equipment is about one week, during holiday season, it's about two weeks. This I can live with.
But sadly Sigma has not made any serious effort to make a camera that I could use for wildlife photography, but one day they will. Also, Sigma makes some of the world’s best pro cinema lenses on the market today.