When using my Sigma150-500m while bird shooting, I have found first that best results are achieved with a solid tripod and a sturdy head. Sweet spot on this lens is F8 therefore, my ISO is in Auto set at 800 with a Max 1600 and no less than 1000 on Aperture. On cloudy days, I push it up to 3200 especially with the D7100. Don't worry about soft corners since you need to catch the object in the center then crop it. The luxury of the 7100 is the 24mp it gives so even if you crop at 100%, you will still have approximately a 12mp image to play with. Now, I know that I will stand corrected by someone on the exact megapixels. All I am trying is to give you an approximation. Lastly, don't forget to shoot bursts on CH and turn your VR off when using a tripod. Good luck and post some more!