Unfortunately, everyone that I know that's tried Sigma/Tamron for nature/wildlife has ended up moving to a high end Canon/Nikon/Sony lens to match their cameras, as soon as they can afford it. Unfortunately again, that can be very expensive. For example, a high quality 100-400mm from Sony/Nikon/Canon will set you back well over $2,000, new. To save money, I generally recommend to friends and students that they look for high quality used equipment instead of new third-party brands. Highest quality used lenses tend to hold their value, so if you by a high quality used, name brand lens for $900 and use it for two-years before moving up to something even better, you likely can sell it for $800. That's not true of bodies, but highest quality lenses tend to hold their values. When I shoot my Sony 100-400mm G Master lens, I'm not thinking about what focal length is weak at what aperture. I shoot without worry at whatever FL and f-stop that I need.