Sigma? Lots of people swear by the Bigma 50-500mm. Also an option is the Sigma 500mm f/4.5, which is more expensive. Finally, for the budget-minded and don't mind manual focus and no stabilization, try a mirror lens. So, the contrast is not as good as a first-rate prime. So, the bokeh is donut shaped. If you are an occasional user, the price can't be beat at under $200.00. Take whatever I say with a grain of salt, because I am a Canonista and shoot with the budget Canon birding lens, 400mm f/5.6L.